It's very hard to actually write good UX from scratch, particularly stuff like scrollbars that require all sorts of relative resizing, drag listeners, velocity methods, second and tertiary controls (keyboards, drag-selection of text) and often need to handle those things after a variety of user interactions without redrawing their content or slowing anything down by constantly remeasuring the rest of the UI.
Because it's difficult, it's often done poorly, which leads to people thinking it should never be done at all.
To your point about games, I find most custom-written scrollboxes in games to be pretty terrible, but we all kind of appreciate them just for their uniqueness, because as UX items they're generally not something you interact with that much. (Unless you're playing Crusader Kings, in which case ... )
I remember being disgusted and offended by Apple's app developer rules two decades ago, which included dictates such as never implementing your own UX elements like scrollbars. We're still debating whether they were wrong. As long as there are bad ways to build UX elements, some people will do them, and this will serve as proof for a few big companies that it should never be done.