Why do basically all phone manufacturers stop at around 5000mah for the phone's battery?

I know it probably has to do with making the phone more unusable as time goes on so people have to buy the newer ones but still, why don't some famous phone companies try to sell a bigger phone battery since it's such an important thing? People would be more likely to buy a good phone that was just as good as a competitor AND had a bigger battery on top of it. Just a few millimeters more could be enough to fit a 7-8000mah battery, maybe even more, and thicker phones are easier to hold imo, I always need a thick cover to make mine comfortable to hold. Also yes I know about the Energizer 18.000mah phone but that is way too much for any user (and it's a brick lol).