The more pixels you cram on a sensor the less light each individual sensor gets, resulting in less light sensitivity and more prone to noise. Also the MP value has less importance than the lens selection. As peeps already stated above, you aren’t blowing up to fill billboard most of the time, and 100MP medium format cameras are expensive. You’d probably hire for that project and bill the client as a line item if you’re doing that sort of work.
So there’s another reason: price. That expensive bit of kit isn’t worth it if it sits around unneeded most of the time.
Also as already stated: storage. I have half a TB of raw files and I’ve only been sporadically shooting weekend sport/motorsport events for 18 months, on a 16MP camera. Most are shared on social media so MP doesn’t matter one bit, the main gain I can get now is a better lens which is what actually gives you the sharp image (as I said above); if you lens can’t resolve fine details then your 50MP sensor will still produce mud images. Which is why on phones it doesn’t matter so much.
Phone sensor is also tiny, which is terrible for noise and pushing ISO. I should know, I shoot Micro-Four-Thirds which is rubbish for introducing noise compared to full-frame. Smaller sensor = smaller pixels = less light gathering = more noise. If what you’re doing is well lit then it’ll be fine though, phone makers seem to know their shit. but unless the lens on the phone is perfect condition and you have decent available light or stablisation… then MP doesn’t matter.