It's just people following trends I think, designers not knowing about WCAG, and designers with decent eyesight not knowing to consider people with poor eyesight. Someone sees some elegant looking site that has grey text and/or thin text, they copy it, someone else copies that etc. without really thinking about what it's going to be like to read a long article with that design.
- From my experience working with designers, many have minimal understanding of WCAG and will only adjust the contrast near the end of the design phase (as in, after the client has signed-off on it) to get it within passing limits if they're told to, and only if it's easy to do without spoiling their design.
- The rules for body text are pretty simple and I'm not excusing it, but WCAG is intimidating and confusing for most I think (ask designers why they don't follow it or understand it). There's loads of rules, the documentation is verbose, and it tells you what needs to pass, but lacks guidelines on how to do this in a way that doesn't change the look of your branded designs.
- I've been in this situation as a developer a few times where I'll tell the designer what specific WCAG rules aren't being followed, and they'll not change anything because they don't know what to do without compromising their design e.g. I'll mention "your main brand color can be used for large heading text, but it doesn't contrast enough for buttons", where the options might include picking a darker brand color (a big ask), a darker color just for buttons (might look weird), or maybe black for buttons (will change the look a lot).
- It's really common for designers to pick bright/vibrant colors for the main brand color too that won't be accessible for body/button/link text, so being told they have to pick a darker color can feel like a big unfair restriction, especially when they see large brands or the designers they follow breaking the WCAG contrast rules all the time. Also, when most brand guidelines don't include contrast metrics for recommended color pairings, you can tell this hasn't been thought about much.
To help with the above, I started working on a tool to create branded color palettes, with the idea that you pick the colors for your headings, buttons, body text etc. that have accessible contrast upfront, rather than tweaking the contrast of these as an afterthought later:
https://www.inclusivecolors.com/