But when you buy Bitcoin, someone is selling it to you.
Why are people selling Bitcoin?
The knowledgeable people are the ones who are now slowly selling out before the bubble bursts. When you are now buying more and more Bitcoin ("investing" as people in here call it) who do you think you are buying them from?
You are buying from the knowledgeable people who are slowly getting rid of their Bitcoin.
People are selling bitcoin for them to gain the money they want to earn. They spend their time to buy and hold it and if they think the time they are in is the best time for them to sell it then they will sell it. That is basically the answer for this question. Also, some of the panicked investors sell bitcoin even though it is not the right time to sell bitcoin because of lack of patience.