出てって And 出て
 

I hear people say "出てって" instead of "出て" when telling someone to get out (e.g. get out of the room) I wonder why?

The てform of "出る" is not "出て"?

First of all, this has nothing to do with the て form of 出る.

When someone orders another person to go out of the room, native speakers don't say 出て in the first place.

We say 出ていって, and the more colloquial pronunciation of that is 出てって.

Another often-heard pair is 出ていけ　+　出てけ, with the latter being the colloquial pronunciation.