It's being worked on :) The plans I've seen use 60 GHz spectrum, which is very short-range (shorter than an average datacenter) and line-of-sight. The latest proposal used beamforming to bounce the signal off a ceiling or wall to further reduce interference among nodes. https://www.cs.ucsb.edu/~ravenben/publications/abstracts/bea... Making this automatic would be awesome :)

Probably because there's limited wireless bandwidth. It'll work, but it doesn't scale - doubly so for colocation, where other customers might eat your bandwidth. The "short distances" would help, but there might not be enough throughput to make it worthwhile.