You don’t see governments or companies using gmail, now do you.

Many definitely do use it. But now that many have moved towards microsoft and/or google cloud services (mostly pushed by the private sector), people are indeed noticing that maybe, it's not the best idea for public institutions to be dependent on foreign corporations.

Why should companies and governments use TweetBook or Snapstargram for official communication when they can host their own instance.

Well because "cloud is the future" and hosting your own instances is not "cost effective".

For the time being, the problem has been that large majority of the people are using these unstable platforms, so companies decided to follow.

Big tech companies have been fighting for the dependency of the private sector for decades. Even before the cloud, there was a dependency on windows, Microsoft office and exchange. Now big tech is selling the promise that "they will take care of everything, you don't need a ton of IT employees who administer everything, microsoft/google will take care of everything".