Are they? How do you store spherical cows? Do you need to roll them out to pasture and then back into the barn, or do they roll on their own?
Why does everybody doing as if I propose something outrageous or impossible?
That said people here are interested in the different ways of solving a problem, if not for anything else but to tickle one intellectually. So yes rectilinear lenses exist, but that does not mean that computational methods are uninteresting or useless. For one thing, one need not purchase different kinds of lenses.
It sounds like you are thinking only in the context of photography. In robotics and machine vision applications you often choose the fisheye lens because they are cheaper than the rectilinear lenses with the same FOV. (if a rectilinear lens is even available in the form factor and FOV you need.)
So what people do in those situations is that they get a crappier lens and they calibrate it so the algorithms know how much to correct for its crappyness. That is where this kind of calibration really shines.