Because there are already male robots. Optimus Prime is a dude. He is not genderless and never has been.
Also, because there have been female Transformers since fucking G1.
Technically, since the Female Autobots were only introduced in season 2 of G1, one could have considered the Transformers to be genderless during the first season (although I thought Optimus Prime having a girlfriend was more than enough to make it clear that Transformers did have gender).
And then, the G1 comics explicitly stated that Transformers were genderless (no Female Autobots in the comic book side of G1), until Arcee was specifically created (by the Autobots) as a response to the question of "If you're genderless, then why do you all look and sound so male?"
It's not clear if Female Autobots (beyond Arcee) ever existed in the canon occupied by Transformers the Movie.
And then, the modern incarnation of the comics even repeated the position of the old comic books and said that Transformers were genderless until Arcee was created as an anomaly, until the slightly-more-modern comics threw that shit out the window and said "Nah, there have always been female Transformers. There are even gay Transformers, if you really want to know."
