Because there really isn't a conversion process in Hinduism. You either are or you're not. In a way, it's not even a religion. See Atheism in Hinduism.
Uhh Hinduism is very much a religion. If you believe the same beliefs (consciousness is fundamental, atman = brahman, etc), why not say you're a Hindu?
There are a number of differences, though. Roberts isn't trying to claim that she was a cradle Hindu (Dolezal found a black man to claim to be her father, with the implication of being her birth father). Roberts isn't taking any leadership positions in Hindu organizations or trying to speak publicly as a Hindu; she's been quoted as specifically refusing to speak about religion. Other Hindus, one would assume, helped her in the process of deciding to convert. She isn't trying to claim any identity other than a person who happened to convert to Hinduism and is practicing Hinduism privately.
Even if trans-race were a reasonable thing (and the reasons that it's unreasonable are far too complicated to get into, and far too off-topic), Dolezal's actions and Roberts' actions would still be very different. Even if you lived in a world where trans-race were somehow legitimate, I'd imagine lots of people would still be uncomfortable about Dolezal's approach to it. But religious conversion, in general, has long been accepted, and Roberts' conversion is extremely normal as conversions go.
It is true that occasionally religious identities that have ethnic identities get complicated; as a random example, the Eastern Orthodox churches in the US have some trouble with this, since the communities overlap heavily with ethnic (Greek, Russian, etc.) identity, and there are occasionally some awkward spots when converts to Orthodoxy start gaining leadership roles in the church (which is reasonable!) and then trying to mold it into the vision of the church that they were looking for before their conversion. But I wouldn't doubt the seriousness of these converts' conversions, even so, and regardless, Julia Roberts is doing nothing of the sort.
"Roberts isn't ... trying to speak publicly as a Hindu"
I'm not sure this sounds like a reasonable standard to join any religion. "If you're a convert, shut up". Sounds like second class status. This stuff is too complicated for me.
I'm not saying it should be a standard, and in fact I would argue that it's a completely unreasonable standard. I mentioned, positively, converts to Eastern Orthodoxy who join the clergy. (One of the best known modern writers about Eastern Orthodoxy is Kallistos Ware, a metropolitan bishop, who converted from Anglicanism at age 24. Nobody would call him second-class!)
I'm just saying that it happens to be true for Roberts, and that this differentiates her case from Dolezal's.
In plenty of cases, the outsider or convert better understands the religion than the native believer. I saw a documentary about Mount Athos, and all the young Greek monks were admitting that being a monk was just a way to get out of the everyday grind. I'm sure the non-Greeks had greater sense of purpose because they had to go further (not just geographically, but spiritually)
Another data point, possibly unrelated: conversion to Judaism. I don't have a source for this, but I have heard (and it has been my personal experience as well) that religious observance is correlated to acceptance of converts. In other words, more religious Jews tend to be more tolerant of converts than less religious Jews. I think this is because you really can't "convert" to cultural Judaism, and the cultural Jews I have known have found the idea of it kind of repellant, similarly to this article about Rachel Dolezal I just read[1]. But in the case of Judaism, if you're culturally Jewish and don't see any value in the religion, all you have is your experience of day-to-day life as a Jew. You can't convert to being picked on for being Jewish. Or black, or Hindu. But I think that attitude is more about building walls than empathy. If someone comes to you saying they're a huge fan of your whole situation, kicking sand in their face and shouting "YOU DON'T KNOW ME!" doesn't make the world a better place.
The situation in Iran is complicated. Zoroastrianism is a protected group there, but it's extremely dangerous to be seen as welcoming converts in the Islamic republic. Simultaneously, to convert away is basically to accede a small amount of political power. I've heard (especially in Religion in Iran[2]) that there may be a Zoroastrian movement taking shape; as converts, those folks would certainly be welcoming of converts, but it's likely to make the established group anxious because if it gains too much attention it will lead to persecution of all Zoroastrians in Iran. So you have two overlapping marginal groups each trying to exaggerate the figures in opposite directions—which is sort of thematic for the kind of contradictions you see in Iran generally.
In the case of Judaism, Israel makes Orthodox conversion more useful since it doesn't accept you as a real Jew if you are a convert to a more weak tea version of Judaism. People who convert are usually stronger believers, so they would convert to the more undiluted version of the religion anyway.
On the other hand, 'cultural' Jews are more likely to marry non-Jews, and absorb them into Judaism that way. And I suppose this leads to cultural Jews leaving Judaism too. I think I read somewhere that the descendants of all the founders of Reform Judaism ended up outside Judaism.
> People who convert are usually stronger believers, so they would convert to the more undiluted version of the religion anyway.
That doesn't follow; converts are very often quite passionate about the beliefs of the group they convert to, and it's distinction form others, but that doesn't mean that they necessarily prefer the version that is seen as most pure by outsiders.
Israel will accept you for making aliyah with any conversion (or none, if you have even a small amount of Jewish blood). Being a "real Jew" in the eyes of Israel may have some cachet for someone... but I don't know anybody who made a decision about which movement to convert in on that basis. I also doubt seriously that the Orthodox movement experiences more conversion than the other movements. It's smaller than either the Reform or Conservative movements.
For making aliyah, you do not have to have an Orthodox conversion. This issue really only comes up for marriage. And of course, Israel has (in their typical fashion) chosen the solution guaranteed to annoy the most people.
I don't think any practicing Jew would make a distinction there. But Judaism predates distinctions of this sort—a fact that I think has irritated Western thought for thousands of years.
