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. Take note that even "whales" that attempt to manipulate markets can also lose money.
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 I guess that explains everything. Regardless of the amount of their possessions, they will never have the full control over the crypto prices. Everything in the market is still unpredictable so whales still end up with price predictions after all, like any other small investors here in the crypto market.