It does the lowest it thinks it can get away with since less bandwidth saves them money. You need to train it that you actually want higher quality by changing it often. I use revanced to avoid the whole issue. (It also has handy 3x playback speed for slow talkers)
Low quality saves cost for YouTube and YouTube thinks that you won't notice the difference between 480p and 1080p on your phone. They don't fuck with your quality on desktops because they know you'll notice that
I mean, if you’re using your phone, you’ll barely notice the difference. The only reason I care about higher resolution is because they’re the only options with 60fps.
On the desktop version of youtube, automatic used to say that the quality was 480p or 720p, but when i switched to those qualities, the video looked much better