They are, however, mass-manufactured, and how to build an Intel motherboard is well-known, there's very little research cost involved. An ARM motherboard really couldn't be mass-manufactured - there wouldn't be enough demand - and you're starting almost from scratch in terms of knowledge of how it integrates with the processor.

Also, if you wanted a socket, you'd have to actually design a socket for your ARM chip, and then sell ARM chips that are socket-able, with few people who actually want them.