Whales have tens of thousands of Bitcoin because they grabbed the opportunity and bought more BTC when price was not very high Bitcoin was not very famous. They HODLed it for long time and that's why they can control(manipulate) the market. As soon as more countries will show interest in crypto, more people will try to buy Bitcoin and maybe then it would be uniformly distributed and numbers of Whales would decrease.

People having tens of thousands of Bitcoin doesn't make it centralized. Decentralized means that no one can stop its circulation except for those who hold it in their wallets. No Govt can control it.