Whales can only manipulate what they have, they can't do that to the Bitcoin. What they do to manipulate the price of Bitcoin is they bought the altcoins they want to pump at less price and this reduce the circulating supply from the holders and few other traders and then they buy back the available coins continuesly until the meet their target before they offload their bag in the market. They don't have the money to buy all the Bitcoin from other trader because it's too expensive to manipulate the price.

If you sum up the total circulating supply of Bitcoin that are in exchanges, they are far less than the Bitcoin that can be found outside exchange wallet and for this, it will be difficult to manipulate the price of Bitcoin as less supply is deposited in the centralized exchanges and this numbers can't be manipulated to play the price of Bitcoin but when you compare and contrast the supply of altcoins in exchanges and external wallets, you will know why it's easy to control altcoins.