My understanding is LoL productions are still trying to make it look like you are watching actual athletes do a sport. If you lock the away in booths it puts too much of a distance between the audience and the players.
OTOH crowd noise giving away what's going on in the game has been a problem since their broadcasts start. It doesn't ruin the game but it's definitely a factor.
That doesn’t account for the fact different esports demand different requirements, just as a squash booth isn’t used for tennis.
CS for example is also Valve but does not use booths. I think a major part of that is that the delay between audience reaction to an event and the event unfolding is much shorter, so the crowd can’t give as much away as quickly.
OTOH crowd noise giving away what's going on in the game has been a problem since their broadcasts start. It doesn't ruin the game but it's definitely a factor.