Android & OEM Skins

So I've been thinking about this today after reading a post on this forum, and I really couldn't find a logical answer.

Why do OEM's feel the need to put skins on their Android devices? I was thinking that it's probably about profits, or to differentiate themselves from other OEM's, but that just doesn't make sense. Maybe it made sense pre ICS, but there just isn't any benefit from it now. Pre ICS Android looked pretty... bad, so skins were understandable. Post ICS Android is beautiful and I don't think OEM's have really shifted well. They have just become used to throwing skins on their phones and it's annoying. The average customer may not know the difference between TouchWiz, Sense and AOSP, but if they did I think we can all agree they would probably pick AOSP without a doubt.

If OEM's feel the need to add their own features, I don't think there's a problem with that. Even if you want to add their own apps or settings, again, no problem. But why change the overall appearance of Android entirely? Stock Android is beautiful as it is, and quite feature-full.

Why do you think OEM's ship phones ONLY with skins?