There are a lot of people who are betting in DOTA 2 and the prize pool is always high compared to other games. Maybe soon I'll try to bet in a FPS games like that, I'm updated in our local tournament but I just don't feel about betting in that game because it is really unpredictable on which team is the best.
No doubt it's unpredictable to bet on esports but if you know the teams you are betting on, their playstyle, their weakness and their strength then it can help you a lot. For example, if you are betting on DOTA2 and observe some teams closely like Nigma, you will notice that player Miracle plays a few heroes amazingly while others not so well. Some even say that Miracle's Medusa is impossible to beat and these things help you make bets because you can see if the players are picking heroes they are comfortable with, then they have better chances while odds aren't much impacted with how the draft phase goes.
Most sports and esports are unpredictable but being able to know the comfortable picks of each players will also helps a lot in factoring on which team will win. Yes, knowing their gamestyle and training of will be the best knowledge to have which mostly can be seen on their streams. But there are times where they play different heroes when they are on and off streams. There are a lot of instances that a hero was picked eventhough you haven't seen them played with it.
I've never tried to bet in a live FPS esports game even though I played a lot of games like Valorant, CS:GO and COD etc.
Playing a game doesn't mean you will be good at betting, that's true, I played a lot of soccer in my college time but that doesn't make me good at betting on soccer. But knowing the sport certainly helps as you at least know all the rules and possibilities of winning and losing.
Being knowledgeable to the game your betting can help you determine the winner of game. On FPS games there are certain strategies where pro players do that can analyze by watching their games. But yes, it'll only help you determine the chances of that'll win but it can't make a professional gambler.