[kang]

If this is so good why aren't cameras using 2 lenses? Wouldn't making zoom better by improving the one lens be cheaper than doing so by introducing other?

I am not mocking this, but am really interested. Because it seems like this idea would have already been tried & if it were good would already have existed mainstream for cameras.

[jseliger]

If this is so good why aren't cameras using 2 lenses?

The literal answer to this question is that most serious cameras have interchangeable lenses; I shoot with an Olympus OMD EM5 and primarily use three primes. A lot of people use zoom lenses, or primarily use zooms.

If you're only referencing cell phones, I assume that there is an obvious trade-off between size and putting another lens in the camera.

[MartinCron]

Cameras aren't limited to impossibly thin lenses, so optical zoom is a lot more practical.