Google has a monorepo in which nearly all of the company's code is stored. The monorepo uses a custom implementation of Perforce. No personal computer could download a whole copy of the repo. So for that reason people need to check out a virtual/cloud copy of the repo in order to do work. This has the added benefit (for google) of not allowing individuals to have code on their device which is bad if the employee loses their device or has it stolen.

There are some exceptions for some teams (like Chrome). And when I was there exceptions for people who did mobile (iOS/Android) development, though Google was moving pretty aggressively towards a world where even iOS code wasn't allowed on the employee's device.