Champion Portraits in-game

Something that has always bugged me is that when using a skin, the champion portrait on the minimap / tab screen / side of screen for your allies doesn't change.

Why?

Most of the time it's pretty easy to tell who is who even if one zoomed in on the face, and on top of that the Loading Screen would actually serve a purpose in letting people associate a skin with the appearance they may be more likely to get that skin because it shows up in more places.

So I'm sure there's a good reason for not doing this, I just want to know what it is, and it seems like a gameplay thing over art (because the reason is obviously gameplay-related).