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
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)