Isn't the easy option to spread the computers out, i.e. not fill the rack, but only half of it?

A GPU cluster next to my servers has done this, presumably they couldn't have 64A in one rack so they've got 32A in two. (230V 3phase.)

Rackspace is typically at a premium at most data centers.

My info may be dated, but power density has gone up a ton over time. I'd expect a lot of datacenters to have plenty of space, but not much power. You can only retrofit so much additional power distribution and cooling into a building designed for much less power density.

This is my experience as well. We have 42u racks with 8 machines in them because we cant get more power circuits to the rack.

yep this was the case for us.

I'm more surprised that a data centre will apparently provide more power to a rack than is safe to use.