Part of the reason for this may be because Google in fact pushed the NLRB to disallow organizing activity on internal emails! https://www.theverge.com/2019/12/18/21028033/google-labor-bo...
Is it legal for a union to use employee info directories to contact employees outside of corp comms?
Which might be for the best: Google controls probably nearly every employee's personal email address too!
Calls between employees will be video conferences over google servers anyway - nobody goes typing old fashioned phone numbers anymore...
Also means work pays for the device, gives you a new one every year, and pays for and organises the data plan including worldwide roaming.
If you break it, they give you a new one in 5 minutes rather than haggling with an insurance company taking weeks...
It's a pretty sweet deal if you trust your employer.