Because the A-Force scene was specifically shot to highlight a girl power moment. If you honestly believe it wasn't then either you're playing dumb, you haven't been paying attention to the current culture war that is going on in pop culture, or you don't understand how filmmaking works.
Everyone lining up behind Cap is just Cap being a leader; that's who he is. It's also meant to give fans what they've wanted for years: to hear him say "Avengers, assemble!" It also included everyone, not just the male characters.
Had T'Challa, M'Baku, Spider-Man, Rhodey, Starlord, and Drax worked as a team for a brief moment to achieve a particular goal, I promise you, the scene would've been shot completely differently. It would've been no different from Clint running with the gauntlet and passing it from one hero to the next, all in effort to keep the gauntlet away from Thanos.
In the case of the A-Force scene, the film pauses to allow the heroines to gather themselves next to one another in a line whilst the camera pans backwards to a wide shot. They momentarily strike a pose, the music builds up, literally everyone else in the battle fighting Thanos' force disappears (right up until Thanos destroys Ant-Man's van), and we're off to the races. The writers might as well have asked to include a giant neon sign above their heads that read "GIRL POWER!"
This doesn't make the scene a bad one. Like Ragnaroknroll and WhoMani said, the scene is harmless, there are those who took pleasure in it, and I say all the more power to them. This film was made for everyone. There's no harm in taking joy in one scene that was meant for a specific audience demographic.