If you remember the part in the episode where Eren was swallowed whole, the Titan's stomach is nothing but a big cavity where all the blood and human bits go. They don't digest them, as Titans need no sustenance. They occasionally just spit stuff back up.
Do you suppose that the Titans without humans inside them are actually what becomes of those bodies? Maybe there's someone or something outside in the world that mutates them, and all it takes is the body, and that's why other Titans are so stupid, because they're literally brain dead, like giant zombies?
I mean, we know that the Titans have no reproductive organs and nobody knows where all the new Titans come from, as nobody has ever seen them mate or anything like that. Meanwhile, take into account all of the mangled corpses the Titans eat.
We know Titans can heal wounds as severe as getting their heads blown off in a matter of minutes, so long as that chunk of neck is still intact.
So then, it could theoretically work that the human corpses could simply regenerate all of their missing and broken limbs, should they mutate and become titans. Perhaps this is also why nobody has ever discovered anything from a dead one, because maybe the body decomposes with the rest of the Titan. Which would mean that every time humanity loses one of their own, the Titans gain another in their ranks. That would be horrifying, it's effectively a double loss every time a human is eaten, at least by a Titan who isn't killed.
Just a theory.