They can’t really do that, mostly because it’s not “just 1 person”.
There are a lot of costs going into maintaining the os, apps, custom software, and training for the employees.
Google is giant, and has a huge amount of money. They can afford to spend the costs of training, modifying software, or developing other software for their needs if it reduces their future costs.
A smaller company don’t have all those funds, they wouldn’t be able to invest as much into switching to Linux and maintaining the custom software or finding new software and training.
When people switch to another software, there is also a period of low productivity, when these same people are still discovering the software, and cannot do everything as fast as before. That is also creating additional costs.