That isn't the reason nor is that a convincing argument to not enforce anti-trust laws against a particular company. So one is allowed to have a monopoly in government software and the rest are not?

Reducing competition is one of the goals of a monopolist and anti-trust laws applies to all, no exceptions.

Don't you think there are other companies competing against Microsoft for government contracts like Amazon, Google or even Palantir that includes software?

Also, Apple isn't some "upstart" either.