Not to step on MB's toes here but I think the issue if just that Transformers feel really distinct from the rise of metahumans as a phenomenon. Even if in practice the setting is actually a kitchen sink, Superman is a space alien, Captain America is a soldier from out of time, Wonder Woman is from an ancient warrior culture, the X-Men are genetic freaks, etc. etc., its still fairly easy for the average person in the setting to look at all of those and pin them all as the same kind of thing, even if they want to quibble about who is a mutant and who is a superhuman or whatever. Whereas with the Transformers, its tough to mistake them for anything but a giant alien robot.
The other thing I think is that I know MB and Sep want to prioritize interaction between characters and that might be hard to achieve when the scale between them is so different. Like, for instance, my story is going to be all about martial arts, and I thought it would be cute to try and teach most every character they interact with a move or two if the story allows, but they're kind of prohibited from doing that with the Transformers at like a basic bodily mechanics level.