1. For far too long, many web sites used sizes that were too small. Body text at 12px or 14px just isn't easy for a lot of people to read in most fonts, even if a 21-year-old designer with perfect eyesight and a good quality monitor has no trouble. But the trend-obsessed design profession tended to stick with what it knew, even when sensible people were objectively pointing out how bad the common practice was at the time. Now everyone got the memo, but the same trend-obsessed industry is fixated on giant text because it's what "everyone" else is doing.
2. On current high-resolution mobile displays, the physical size of text for a given CSS size is often smaller than on a desktop or laptop screen. Lots of people are advocating so-called "mobile first" design, and so larger font sizes follow to improve readability at the default size on mobile devices.
3. Web fonts are all the rage, but many of them just don't look very good, even the ones you get charged to use. Often this is because are trying to emulate print fonts that were never designed with a relatively course pixel grid in mind, unlike classics like Verdana and Georgia, the newer Microsoft C-fonts, and analogous native/UI fonts on other platforms that were very much designed with on-screen use in mind. That makes it hard for a lot of web fonts to look good at small sizes even under ideal conditions. And conditions usually aren't ideal, because often these fonts (again, even the commercial ones) are hinted in ways that lead to a poor appearance on-screen for at least some users. This is not helped by the very different renderings that the "same" font can wind up with on different platforms. These fonts don't look so bad on high-resolution screens like newer smartphones and tablets or Apple's Retina laptops, but to look even acceptably good and moderately distinctive on the lower-resolution screens of desktops and laptops they need to be scaled up to see more of their details.
On top of those factors, probably most users now understand and routinely use zooming and it's supported in every major browser, so setting a font too large isn't a huge risk to usability the way setting a font too small used to be. So we have a correction to a long-term trend of using fonts too small, a trendy industry that has overshot the mark, and no immediate compulsion to fix it because users just zoom out exactly as you described.
1. For far too long, many web sites used sizes that were too small. Body text at 12px or 14px just isn't easy for a lot of people to read in most fonts, even if a 21-year-old designer with perfect eyesight and a good quality monitor has no trouble. But the trend-obsessed design profession tended to stick with what it knew, even when sensible people were objectively pointing out how bad the common practice was at the time. Now everyone got the memo, but the same trend-obsessed industry is fixated on giant text because it's what "everyone" else is doing.
2. On current high-resolution mobile displays, the physical size of text for a given CSS size is often smaller than on a desktop or laptop screen. Lots of people are advocating so-called "mobile first" design, and so larger font sizes follow to improve readability at the default size on mobile devices.
3. Web fonts are all the rage, but many of them just don't look very good, even the ones you get charged to use. Often this is because are trying to emulate print fonts that were never designed with a relatively course pixel grid in mind, unlike classics like Verdana and Georgia, the newer Microsoft C-fonts, and analogous native/UI fonts on other platforms that were very much designed with on-screen use in mind. That makes it hard for a lot of web fonts to look good at small sizes even under ideal conditions. And conditions usually aren't ideal, because often these fonts (again, even the commercial ones) are hinted in ways that lead to a poor appearance on-screen for at least some users. This is not helped by the very different renderings that the "same" font can wind up with on different platforms. These fonts don't look so bad on high-resolution screens like newer smartphones and tablets or Apple's Retina laptops, but to look even acceptably good and moderately distinctive on the lower-resolution screens of desktops and laptops they need to be scaled up to see more of their details.
On top of those factors, probably most users now understand and routinely use zooming and it's supported in every major browser, so setting a font too large isn't a huge risk to usability the way setting a font too small used to be. So we have a correction to a long-term trend of using fonts too small, a trendy industry that has overshot the mark, and no immediate compulsion to fix it because users just zoom out exactly as you described.