You can compare the house edge of regular casinos in real life versus the house edge of online casinos. Online casinos just charges 1% or near that amount but compared to real ones, they charge about 5% or more.

Also, remember that even if these casinos are online they still need to pay for a lot of stuff to make them work. It's not like they are running everything for free.

I hope you do understand that betting even without house edge does not guarantee you will win in the long run and that the casinos will not profit from this. Statistically, the casinos have an edge over their gamblers, that is why it is called house edge. But if you analyze an infinite number of games, then you'll realize that there is a chance that the gambler will lose as much as an unlimited number of times in a row. However, this is also true with the house. The only problem is, the house still has more money than the gambler and the house as imposed a limit to the amount a gambler can bet.