but if the whales owned 51% of bitcoin, can do it?
A "whale" can move the markets to a certain extent, but it doesn't give them overall control over the markets.
Also, no "whale" owns 51% of the total supply. Most of the wallets with huge bitcoin holdings(though definitely nothing close to 51%) are exchange cold wallets.
if happen, does decentralization not exist?
Bitcoin is proof of work, not proof of stake. So even if a single person held a huge chunk of Bitcoin, that wouldn't give him special powers over the protocol.
https://www.investopedia.com/terms/p/proof-stake-pos.asp