The laws of physics impose limitations. Stuff takes room. Zooming involves moving a prism inside an optical structure. People don't just want zoom, they want a usable camera, too, which means decent apertures, which means more glass. On the other hand, people want their phones slim and sleek. Cell phones can take good pictures in the right circumstances but those are limited and will always be compared to larger cameras with more technical capabilities. And we haven't even got into filters.