Not sure whether you were serious there, but:
- The unused pins cost money to make, which has to be paid by the customers eventually.
- Where should the "unused" pins be connected to, if nothing is there currently? Any general-purpose expansion port is most probably outdated by the time it could come into use.
- How many unused pins do make sense as reserve? What if you overprovide 100 and suddenly need 101 more for the next revision?
- Technology evolves. Pin layout may have to be changed for technical reasons (LGA2011v3 has the same number of pins as v2) or things like e.g. optical interconnects become mainstream that weren't possible when the socket was originally conceived.
- Many companies make good money on recurring design changes, so why bother?
I was serious at first, but then I realized it it a bit more complicated! Now would be nice if apple would design a cpu tray that features 2011 socket! that is more possible to do but even if it was easy they wouldn't! they would want you to buy the trash can!