This is easy, he keeps lying to you since you let him. He knows you will just keep being there, and that he can get by with it. So why should he stop, he has nothing to lose in his mind.

So my question is, why do you keep forgiving and why do you let him keep coming back and why have you not dumped him.

he has lied, he is lying and he will continue to lie. You let it happen.

So stop wondering about him, and wonder why you let it happen.