Because it is one number that describes it all? It is, also, your limiting factor. You have x available power for running chips and cooling, regardless of what kind of chips they are or how efficient they are. They used "H100 equivalents" to describe compute once, and people misinterpreted that, and complained about it as well.
Otherwise, you end up saying something like 100k TOPS of compute plus 150k BTUs of cooling. Which really isn't any more meaning full. Especially since they are installing NVidia H100s, Tesla HW4 chips, Dojo 1.x chips, Tesla AI5 chips, probably AMD xxxx chips, etc.
And you don't only need power for the chips and cooling, you need it for the networking, storage, and all the other necessary ancillary services/devices.