Thin scrollbars work by being transparently overlaid over text and grow in size on hover, without bothering the main layout. Browsers can’t do that universally. So what you get is a windows/linux-style static scrollbar of an annoyingly small width. Or a js abomination that partially simulates it and stops working randomly on some mobile devices.
Sure, you may blame designers. But these original Apple scrollbars are actually fine and they look good. The problem is where it was for the last 20+ years: we are using slow and clumsy 40MLoCs as an applications and media platform. They need TENS of years to add support for primitive things like flexible boxing or grid layouts or linear layout constraints (oh wait, no). So there’s a chance we’ll get these overlay scrollbars in 2040+.