it's not just that the team brings the danger of lacking focus, though that's a big one. Main characters are not the only way to unify the story, but they work extremely well, which is why they are overwhelmingly the most popular. Uniting the teammates in purpose is hard enough. Uniting them in motivation is extremely implausible and rapidly leads on to the next problem. . . .
When they all are on the team, it's really very noticable if you don't characterize them all; they are all in this together. But it's hard. There is only so much space, and characterization is far from the only thing you need to do. Insofar as they are alike, it raises questions about why duplicate them.
RPG based works have the problem a lot, though a lot manage to work about it, either by scrapping the team or by making the leader the main character and subordinating the others more than players would let a DM do it. Superhero books, too, obviously. Notice how the Justice League and the Avengers were both assembled out of heroes that already existed -- and had their books and characterizations.