Probably because online casinos are less expensive to launch than real life casinos, or simply because more gamblers are using online casinos nowadays, since it is something available in most places and countries compared to real casinos, where they first need a license and huge capital money to build that place.
When it comes to the fairness, I believe that machines and real table games in a casino are really hard to notice their real outcome, even if they put or announce a number, clearly, nobody can open that slot machine to make sure of that RTP. When it comes to algorithms and verifications of online gambling, every gambler can make sure of it with verification of codes.

Personally, I only played in online casinos and gambled online, however, real life casinos, seems more interesting. The only thing that going to make you gamble in real casinos is probably the better outcome, easy way to get your money, and also the entertaining experience.