You don't have to invest into a fluctuating market if you don't want to. People invest into bitcoin for two reasons, one they believe in the project itself and how big it could get, two they believe they can get rich over night or at least faster this way than any other traditional method.

Markets do go crazy time to time but that also means you can spend a little amount on some small coin and come out a millionaire one day, it doesn't have to happen but if you check coins like IOTA and their ICO prices and than their all time high prices you can see how much someone could have made from it if invested at the right time and got out at the right time. That is what people want to achieve, they want to turn 100 dollars into a million dollars. Not many can do that and not many has done that before but there are few people who have done it and that is the motivation for others.