Here’s my hot take: because of the deeply held cultural belief that screen time is bad and books are good.
Certainly there are truths behind that, but I think we need to realize that videos and even video games can be informative, uplifting, moving, etc, just like books. And conversely books can be a huge waste of time.
I think it also helps that, with reading, you usually have one book in front of you. There are no "related books" vying for your attention in the sides of the page. This is more true for physical books.
Re: "deeply held cultural belief that screen time is bad" - I agree that's likely part of it, and that belief probably isn't as rooted in fact as it might be.
Not sure I can say I've experienced a computer game that I could say has come close to being as enlightening or life-changing as a book (though I have met people through playing them that have changed my life, which counts for something).
TV is entertainment, and certainly the free-to-air stuff only exists because it keeps you glued to the screen long enough to absorb the ads that pay for it, and even relatively low-brow stuff (think, uh, Two & half Men, or I dunno, Gossip Girl) can still be oddly addictive and a huge time suck in a way that poorly written books etc. never are, because I far more quickly put them down and find something better to read.
I think it’s just like the difference of being spoon fed some yoghurt compared to having a full course dinner. That being said I still find YouTube better than Netflix, or worse, TV in that regard - at least you chose the yoghurt and some of it is actually pretty healthy (e.g. some of the top notch educational content).
