They are small and thus take less space on the desk, or in your bag (perfect for traveling and on-site-work). And not all keys are necessary for everyone. Numpad is a big part with little use for many people. And the newer generation of keyboards is often programmable, so you can remap them on very sophisticated levels on the keyboard itself. This reduces the demand for more keys even more.

For example, on my keyboard I have no dedicated cursor-keys, but they are mapped to hjkl and wasd with a thumb-key. Function-keys are similar mapped to number-keys with the same thumb-key. This makes writing from homerow much faster, and I need to move my hands not so often. I basically just must my fingers, instead of the whole hand.