It is a psychological thing. People own bitcoin to sell it for cash (hopefully more than what they paid). I would say over 90% of people who buy bitcoin do it for this reason. That is what makes it a bubble. The reason the price increases is because more people are convinced that if they buy it, they will be able to sell it in the future and make a profit. This system works as long as the pool of people that "speculate" in BTC or any altcoin increases. You may say gold is the same thing. And in some ways, it is, but the difference is that gold has a value other than speculation. BTC does not. When you measure the success of buying BTC or any altcoin you measure it by the amount of fiat you had before you bought it and after. People buy gold for the same reason but many also buy gold as jewelry.
Any commodity like apples, oranges, even houses, crypto, stocks, etc... can become bubbles if the majority of people who buy them are doing it because they believe the pool of people who buys them will increase causing the price to increase.
For example, the majority of people who buys apples do not buy them because they believe the price will increase and they will be able to sell their apples to someone else for more money than they paid. Most people buy apples to eat them and then they're gone and the supply decreases. But if all of a sudden over 90% of people who buys apples do not want to eat them but sell them to somebody else for a profit, and the person who just bought them is buying them to do the same thing, then apples would be in a bubble.
Look at it this way. If I gave you 1000 apples and told you that you can never sell them. There would be some intrinsic value because you could eat them. If I gave you 1000 pounds of gold and told you that you can never sell them, fine there is some intrinsic value because you could make jewelry out of them and wear it. If I gave you a stock, say 50% of Berkshire Hathaway, and told you that you can never sell them, fine there is intrinsic value because you get millions of dollars of dividends a year. If I gave you a Lamborgini and told you that you can never sell it, fine you can drive and use it to go places. IF I GAVE YOU 10 BTC AND TOLD YOU THAT YOU CAN NEVER SELL IT. YOU HAVE NOTHING. THERE IS NO INTRINSIC VALUE.