You're confusing colos and first-party data centers. A colo mostly relies on its customers for installs and maintenance. Google, MS, FB, Apple etc own the entire building so all the functions are local: server install and maintenance, network install and maintenance, generator maintenance, electricians, water treatment in some cases... That's very easily in the hundreds if not thousands for big sites.

Count the cars here: 36.242186561973, -95.32619746715035