But they aren’t really “mech pilots” as a general rule; most of the machines they operate are vehicle-based, when they aren’t traveling on a flying pirate ship or battling on top of dragons.
They tweak the suits to tie into whatever sub-theme is current, while still maintaining the “ninja” motif. On a toy level, the suits are the only thing that really makes the line “ninjas”; everything else is a clever mix of fantasy, sci-fi and whatever-punk (cyber, steam, bamboo, etc).
I imagine that’s what’s helped maintain the theme’s evergreen status; there are elements that appeal to all sorts of interests.