Board Thread:Manga/@comment-29900084-20161212032115/@comment-30455198-20161212080231

They are Eldians, Marley can't become shifters. We are not sure about Armin yet, because we have only seen his mindless Titan form. He might as well get skinless like the Colossal Titan. And I think Frieda also was skinless.

But I get what you mean. I also wondered why the warriors had skinless Titan forms. Though, the Beast Titan isn't skinless, and we don't know about Marcel. But still, it applies for Reiner, Bertolt and Annie - the three "active" warriors (Annie is not as active anymore, but she was just a while back).