There’s a good reason they are done that way, namely that few people ever need to directly interact with them. People instead scroll content with their mouse wheel, touchpad or via touch devices directly, and hence the scroll bar is relegated to a pure indicator of roughly where on the page you are, which for many pages isn’t even necessary.
That being said: If you set Windows 11 to “Always show scroll bars” - which you would if you need them as actual input surfaces - then it also signals all apps to show them at full size again. Which Firefox in fact does, as I just found out.