I think it’s a size constraint. Bigger sensors also require larger lenses. In regular DSLR world a micro four thirds sensor, the sensor size is half the size of a traditional full frame sensor. The lenses are also half the size of a fullframe camera’s lenses and require far less glass and weight.

We can fit 3 smaller sensors with three smaller lenses in a smartphone. Trying to make any of those sensors bigger will require more space than a phone can provide.

I’m no expert just a hobbyist take my word with a grain of salt lol