It’s not obvious for me why wouldn’t they install better cameras. For me camera system is one of the most important parts of a smartphone specification. Don’t those makers realize that there are people for whom those things are important?
All this to say: you can't always serve everyone.
Judging by many, many reviews of smartphones, most people do deeply care about the quality and features of their phone's camera, and it's very likely to be the only camera many people have.