Again people bringing up movies like The Magnificent Seven are forgetting that those are very different types of films with very different contexts.
And as for "well we can introduce them via Diana and Bruce going through their files" that was one of the most mocked and derided parts of BVS, and rightly so imo. And they really to try and do that kind of laziness again do they?
Totally different situations. BvS featured an out-of-place scene of Diana watching a few short "trailers" for Flash, Aquaman, and Cyborg that didn't really impact BvS's primary story. JL will actually feature their true introductions. Showing Bruce and Diana review "Arthur Curry -- the Aquaman" and the kid with "organic and biomechatronic body parts -- a Cyborg," and then actually GO OUT to recruit them is totally different.