BGA balls are made of solder, and they'll cold-flow under pressure. So any socket meant for durable use in production equipment (as opposed to prototype or test purposes) is going to require a lot of small, delicate, and expensive spring contacts that provide a secure connection with very little force.

Also, sockets in general are perennial problem areas for signal integrity and EMC.

It is probably best to think of the motherboard as a monolithic hunk of material, going forward. Too many reasons for the trend to go in that direction.