Well, I guess it's because they dont take this seriously. They're laughing about themselfs and do it to have fun and make us laugh with them. I think they don't care about what we draw about them, they're just glad that we love them.
I honestly love it when they do that in their videos, they're so at ease with the shenanigans of their fans. That's what's great about them. Heck, Mark once commissioned the fandom to draw... stuff... with Yami and Slendy. I wonder which video that was.
Oh, it was the one where they played Slender together and Yami leaned over a table and after that he got caught by Slendy who was directly behind him xD Mark asked us fans to draw that situation with Yami as a nurse saying "Oh, stop ist Slendy~" xD