Is there centralization in the decentralization system of bitcoin? I understood there are few BTC mining farms in the world. This mining companies are not distributed across the continents, let's say each continent should have one mining farm.
First, I think you're mistaken mining farms for mining pools, there are not just a few mining farms, there are a lot of them and they quite distributed themselves across the globe. Second, what would your distribution achieve with decentralization, one farm in each continent is better than three farms in Europe, one in China US, and Canada? And if there would be one farm in Africa let's say in Nigeria, how is that evenly distributed when Nigeria has one farm the rest of the 50 countries have none?
What you're trying to do here is actually centralized planning, there are a few counties who tried that last century, deciding what to build in each city based on population distribution rather than economical reasons, so that all regions would have their own industry of everything, and we all know how this ended.
Is it therefore not wrong to say that there is an iota of centralization in the decentralized system.
As you can see, it is wrong!
For the evil government part, when one government starts building a farm that would be able to centralize bitcoin, again things don't work like that, probably you've mistaken a 51% attack with somebody gaining control over all the hash power of the network, and that's pretty impossible. Not that a 51% attack right now is feasible in any way unless a G7 government is bent on doing that! Just the power required for the attack is bringing the list down to 30 countries as they don't even have the necessary electrical power capabilities to host such a farm even if they cut power to all the people in that country.