|Art by Brad Green|
That's the main point here. Batman did not form a partnership until he was able to control their learning.
Every sidekick Batman has ever had has been taught closely by Batman himself. He knows how they think, showing them how to react to situations. In a battle, Batman is confident that they have the skills necessary to provide support. But, in a team setting like the JLA, Batman doesn't have those guarantees.
Granted, the JLA is comprised of DC's most powerful and talented heroes. They have so many different skills and knowledge-bases to take on whatever they need to. I'm sure Batman trusts their abilities, but does he trust them to make the right decisions?
For someone like Batman, I wouldn't think so. Batman would know the way his teammates thinking, putting in the time to study their past actions. But his teammates wouldn't be so studious and wary of the other JLA members. Batman would understand their psyches, but they wouldn't understand him. This would prevent him from joining a team like the JLA.
I'm well aware that Batman has been a member of almost every JLA team in the DC Universe. However, this is mostly a ploy to sell the series. Would the JLA be interesting without Batman? Absolutely. Would it sell as well? Probably not. Despite the fact that Batman is a full-fledged member of the JLA, judging by his psychosis and trust issues, were he real, his being a member of a team is completely unbelievable. Sure, Batman can be a team player, when he's in control. With the JLA, Batman needs to relinquish much of that control, which I doubt he would be comfortable with.