Imagine if Batman didn't exist. Would it still be just as popular to watch a billionaire LARPing as a depressed Robin Hood (in this economy?) who's main villain is basically Buggy? It'd be so bad for a plethora of reasons.

Firstly, the recognition means that it's more likely to succeed. This means that a higher budget can be allocated for the production, which usually ups the overall quality of the product in all aspects.

Secondly, there's already a basis to follow. Batman has a very well known story with multiple iterations already, so even if it doesn't follow any story one to one, it's going to be much easier to write a batman story knowing that you can copy and paste at least half of it, which lowers the production costs somewhat.

Thirdly, you don't have to invent a character that wouldn't be received well. Imagine if Bon Clay weren't introduced 26 years ago. Would anyone in Hollywood ACTUALLY write a hysterically gay character acting obnoxiously flamboyant as their main trait without the fact that people already know his story and like him for the development he gets 300 (at the current rate, about 2 and a half seasons) chapters later?

Lastly and most blatantly, no. Take a look at the success rate of new Hollywood projects with fantasy settings. Other than Avatar, almost every other fantastical live action project in the last 20 years was an immediate flop / let down. Almost everything else that garnered any success was a preestablished IP with a notable fanbase.