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.
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.
Exactly. That would be centralization, a central authority deciding which countries/continents need to mine more or less.
The free market is the most efficient system we have, so far, to let miners decide where to place their operations. There is no central authority asking them to move to china. They have economical reasons to be in China or Canada or whatever: cheap hardware, cheap and abundant electricity, low temperature (canada) and so on.
Mining is not centralized as it looks like. A mining pool isn't a single computer or a single farm. A mining pool hashrate may be distributed across the globe.