Well it's not like they really like... actively stole them. That might have been apoor word choice on my part. But they have this hole on their backs, it's where the Ninjask breaks out during evolution. And if you look into that hole, your soul gets pulled out of your body. And you just go... lifeless. [He pauses, seeming to leave himself for second or two. He's over the incident by now, but he still can't honestly say he likes reliving Rose getting her soul sucked out.] Like you could probably almost mistake the person for sleeping if it weren't for the part where they just stare blankly into space and don't move or react or do much of anything at all.

I didn't look into their backs so I don't have any first hand experience as far as what it's like in the hollow so to speak, but I know a few people who did. The described it as being almost like a really weird and disorienting dream. Or a nightmare, in some cases.

Lucky you though! Morty didn't do anything like that for me... though I was kind of way overleveled by the time I finally made it to his gym so...and they didn't do the Elite Challenge thing back then.