Because often it doesn't take someone to complete their sentence for their point to be made. If you pay close attention next time you are in a conversation with people you know well, you will often know what someone is going to say before they have finished saying it. Personally I hate being interrupted and also always wait my turn to speak but at the same time I can get impatient waiting for someone to say what I already know they are going to say.
That's true in some cases, but untrue often enough that I disagree.
In my experience, interrupters frequently assume something entirely different from what was actually being said, or even cut off the speaker before a "but" and wind up responding to the exact opposite of the intended point. This is needlessly annoying and confusing to almost everyone involved.
