Because it has to be about goku. No matter how little sense it makes or how narratively retarded it is...shit like this has to ultimately be about goku.
Look at that scene
Broly had just beaten vegeta in fucking SSJB form and goku in base is fucking beating him with his sweet kungfu skillz before he powers up.
Full on goku puff up that makes no logical sense within context of all of fuckign dragon ball.
Hell goku vs frieza makes even less narrative sense if you really think about it.
Vegeta should tell his wife he'll be leaving, kiss his daughter good bye and give his son a replica of his fathers amulet and go into space right now looking for frieza and trying to take down his fucking empire. Not fucking around with tardku or being the "you're sick of goku we know we know but her'es vegeta" spoiler.