how can it be cheaper? people will spend the same amount or even more considering that is more easy to spend more since it's digital

It's all hypothetical of course but I know Vegas has some high table/game minimums and these markets can be pretty cheap if you just want a piece of action. Also, eliminates the cost of actually traveling.

Again, no idea if anyone sees this as a true substitute or not. My guess is not as Polymarket bets don't feel entertaining at all (IMO). So it's not filling that void for anyone, but it hypothetically could.