Why are, generally, used different camera in rear and front cameras of a smartphone?

It's a technological limitation? It's possible to do a smartphone with same camera system (Lenses and sensor), in rear and front sides? It's possible to use flat lens, instead spherical or aspherical ones?

Pure price/performance and market driven. People don't care if the front camera is lower quality so they use a cheaper/smaller one. Same with the lens type, the lenses used are those that the companies think people want, with price also a concern.

Does the front camera need to be as good as the rear one, given the front camera is primarily for taking selfies and uploading them to Facepage, or whatever it's called. I must be old. I can remember when phones only had one camera. My Android smartphone probably doesn't have a front camera. I'd have to check to refresh my memory as I rarely take photos, but it's ancient for a smart phone (it must be 5 - 6 years old).

Size is the biggest factor. As you can see by looking at a phone with camera's on both sides the one in the front is minuscule in comparison to the back camera whose optics require that big bump on the outside. The bigger the optics the more light can enter the sensor, of course the sensor size is also larger at the back also. The front camera imo doesn't need to be as good as it is primarily for video calls and self-obsessed selfie taking narcissists.

No, I believe it works like this with a flat lens. Forget thinking of a flat piece of glass. The curvature is controlled by varying the nano pattern in the material. It is truly flat over the entire surface. Something like: Code: