1. There is enough room. Works fine for media controls where the bar is near the bottom of the screen, but not for sliders in most other places
2. The control is not close enough to the bottom of the screen for swipe up to trigger the multitasking action
There are many other reasons to have fine-grain control for sliders and imo “hold longer for finer control” works in more places than “slide up for finer control”
YT also has Premium Controls whcih is a widget but I think it's experimental and not on mobile yet.