People are buying Bitcoin because they expect other people to buy it from them at a higher price; the definition of the greater fool theory. Someone responded to me on Twitter by implying the fools were those who were not buying; everyone who did so had become a millionaire. But it is one thing to become a millionaire (the word was coined during the Mississippi bubble of the early 18th century) on paper, or in "bits"; it is another to be able to get into a bubble and out again with your wealth intact.
https://www.economist.com/blogs/buttonwood/2017/11/greater-fool-theory-0