[dom0]

Many of these will be ground and power, which directly terminate into plane(s) and aren't routed. Things like memory buses are put onto the socket such that they are routable to the memory sockets (the location/orientation of the memory sockets is prescribed by the processor socket!). Other interfaces are arranged to "make sense" as well.

I expect it's pretty much the same problem overall as with the (much smaller) pinouts of SoCs and such.