[PeterisP]

Because we are unable to do that, and we've tried for decades.

There are all kinds of things we're doing (e.g. rewriting things in memory-safe languages) to make it less likely for an attacker to become able to control a jump to somewhere, however, we don't expect to fully succeed any time soon, and this is defense in depth against cases when attackers once again do find a way to control transfer to some arbitrary gadget.