They can, but
1. They won't able to buy all bitcoin due to people who lost their private keys & bitcoin maximalist who'll never sell their Bitcoin
2. They've learned history and know the risks.
Imagine what would happen if
Silver Thursday and
Hyperinflation in Zimbabwe event happened at same time in a county

While perhaps the deal with the exchanges of your country would work somehow, but things get complicated once you'll try to buy up all Bitcoins on exchanges of other countries.
Before the other's say it, let me say it first
5 Eyes, 9 Eyes and 14 Eyes