«They’re ostensibly based on toys (although I can’t imagine the bulk of the target audience is old enough to have played with the toys or watched the cartoon)»

I wouldn't be so sure about that. Transformers has always been (and was started to be) one of those perennial brands of toys (like HotWheels) with "new" toys every year. I believe there has been a near-continuous supply of the toys since the franchise started, even years without a cartoon (and the toys have always lead what appears in the cartoons). Even then, there have been very few years without a cartoon for the brand: the Beast "saga" lasted nearly a decade, for instance, and Cartoon Network was airing new episodes (in what the fans call the "Unicron trilogy") as recently as 2006.

If there's nothing but childhood nostalgia at play here, it's a deep multi-generational vein.