All of them consist of things you can get for really cheap but worthless. For example I wanted to check out cryptokitties for a while to see if there was any chance of making a lot of money from it with sheer luck (breeding gone right) but honestly even the potentially good investments are already taken and expensive to begin with.

All those cheap kitties you can buy are really worthless and doesn't really worth buying to begin with which makes it harder to sell as well, so you are stuck with some kitties that doesn't worth anything, putting the gas fee on top of that would make you lose money instead of making any. Hence, if you are going to get into these type of games you need a bit of money and a lot of patience and study and research them very carefully to actually make that type of money.