If you could leave, you'd never be trapped in a long game. You would enjoy every long game. The ones that suck wouldn't last.
Root problem: the game requires a fixed number of human players, from start to finish. If bots worked then you could just take the L and quit. Or safely eject someone who's being a total cock. Or possibly even split the game in two, so both the "fuck this" and "fuck you" groups see everyone else replaced with bots.
Bots don't have to be good with every character. Bots don't even have to play by the same rules as humans. They just need to be balanced. Which you'd figure these developers are really really good at, after fifteen years of pouring new characters into these games.
Individual scoring would be almost as powerful. A high-level player with a low-level team should ideally be scored on their skill - not a binary win / lose condition. Especially if half the players are guaranteed to lose. Long matches provide oodles of time to evaluate. And if bots work at all, the game can quietly run simulations from snapshots of the ongoing match - checking if players did better or worse than a player-like script would, and by how much.
Compare sports. You have a regulation basketball game. On one side is the 2023 Miami Heat, minus Jimmy Butler. On the other side you have the AZ Compass Prep Dragons, plus Jimmy Butler. The Dragons' chances of winning are approximately diddly over fuck. But a talent scout watching those high-schoolers get smoked 132-15 can still recognize which of them are doing especially well under the circumstances. And Erik Spoelstra can still give Tyler Herro side-eye for ever missing a free throw. Despite a blowout loss, every individual can be judged for how they played, both in terms of independent actions and productive teamwork. (This new kid at Arizona, Jimmy somethingorther, is really good.)
Yet in a video game - where every moment can be scrutinized frame-by-frame, and statistical analysis is so easy you'd think this was baseball - there is only total victory and utter defeat, and only for whole teams. Everything from Smash Bros to Overwatch has little trophies to hand out for leading performance in a bunch of arbitrary details. So why doesn't a loss caused by one feeding troll count as 90% of a win for the players who almost eked it out in spite of them?
More importantly: why doesn't the game make it feel like they were doing good, when they were doing great?