Eh, whether it comes from stereotypes or not, I don't really agree. It's just as easy to say that a kid can relate to a fellow human more directly than something that behaves like one.
Even the US Transformers series introduced human POV characters despite already having kid-appeal characters in the main robot cast. This was a convention pretty much every series of the era also used, but the fact that kids were still given that IN the series where they could also relate directly to the anthropomorphized robots says a lot.
There's something to LBD Nyetrayn's idea that the piloting thing may be more aspirational in the culture or maybe even in general -- rather than just "relate" to them, kids can want to be or become them, having these cool vehicles and mecha at their disposal. (Both are valid ways of connecting to these properties, of course.) Like wanting to become a race car driver or construction crane operator or astronaut. I'm not about to draw assumed connections between the appeal of one or the other and degrees of maturity.
It's pretty interesting how once Takara had more of the storytelling role for G1 animation, it didn't take long before the series started incorporating that approach, from the Headmasters being pilots to having human/oid pilot partners.
