Why do films continue to do this?

In this case it was about turning the table on the movie audience. At the beginning of the movie, the audience assumes he was the one being cheated on and lied to. Standing in the rain at the door, when he asks "Is he there?" and "I promise i will leave you both alone." the audience is expected to have sympathy for a man who has been jilted. The character Beth says "I feel sorry for you, i really do" leaving the audience to feel her cold and unsympathetic because the audience assumes she is the liar.

Then as the movie ends, the table has been completely turned and the audience sees him thru different eyes, as the liar and the audience realizes she really does feel some sympathy for his pathetic lack of character and integrity. This leaves the audience feeling they have gained some insight having sat thru two hours of pretty bad writing, and a "B" movie in general. That is, they wouldn't feel quite such a waste of time.

Well, i think that is what they intended anyway. You decide if it worked you. but the movie is barely above a 5 in 2015, so i think it didn't for most, or was not enough.