Sensor size is super important for resulting quality, that's why pros still lug around huge full frame (even if mirrorless) cameras and not run around with phones. There are other reasons ie speed for sports but lets keep it simple (also speed is affected by data amount processed, which goes back to resolution).

Plus higher resolution sensors have this nasty habit of producing too large files, processing of which slows down given devices compared to smaller, crisper photos and they take much more space, even more so for videos. That's probably why Apple held to 12mpix main camera for so long, there were even 200mpix sensors available around if wanted.