There's already been lore implications that warframes have some sort of personality of their own. I think it was in the codex entries for Rhino and... Mag, maybe, that were big ones. Also supported by 'frames having sexes and unique animation sets, which often convey quite a bit of character.
That's why my own opinion has always been that the creation of the original warframes involved some sort of fusing or subsuming of an individual, and that everything else is copies of that (Primes) or copies of copies (contemporary frames).
Well then how do you explain the frames becoming invulnerable to damage when the operator is out? 0_o
Because it would be really shitty gameplay if using an ability they're trying desperately to force people to use left your warframe totally vulnerable to damage in addition to deactivating defensive abilities. Let's be real: people already never use operators when they're not forced to. If using the operator meant that you were basically guaranteed to die, do you think that would change?
I believe there's also a line of thought that "damage" to the 'frame is actually just force-feedback transferred to the operator, either as physical pain or reduction in the quality of their connection (maybe both), hence why you never need to repair frames and can instantly revive on your own--treat the latter as being something like a hard-reset on your link. Thus, when you're in kid-mode, you're using all of your bandwidth to maintain the projection, so stuff hitting the frame doesn't even ping on your senses.