To maintain control over critical communication technology.

The problem is that 5g wasn't designed to be secure against malicious carriers & governments. It could have been, and modern cryptographic protocols used over it can be.

The assumption that you can trust communications infrastructure is outdated. It must not matter if an adversary has back doored a router or cell tower or similar to send copies of all traffic to them, since the data should be entirely encrypted (except for some minimal routing information). The 3GPP designed the 5G standards to allow back doors, so we get back doors.

What if the same company also provides the cellphones and computers used to do the encryption of the data being sent?

Then you're programming Satan's computer[1] and need to start doing things like using a better trusted OS & compiler. Or go to a different vendor. Since end-user devices need to swap around a lot this tends to be a lot easier than replacing the underlying infrastructure devices.

[1] https://www.cl.cam.ac.uk/~rja14/Papers/satan.pdf

[oops, misread comment.]

Making our own 5G equipment != Actually buying it.