@Wintersnow17
Don't know why they do it OP. But I have a story that may cheer you up.
My friend's husband had an affair with a woman at his workplace for 10-12 months before she found out. Everyone seemed to know about it except my friend. She was devastated, and thought their marriage was strong, and that he would never, ever do it to her. 23 years they were together (1 daughter aged 20, at uni.)
The OW was all over him like a rash, and facebook messaged her saying 'you have never treated him right, so you can hardly blame him for wanting me.' And the OW told everyone at work, and her friends, that she was a lousy wife, she never cared about him, and they hadn't had sex in 5 years. (None of this was true.)
Long story short, just before Christmas 2016, he packed everything up and left my friend for the OW, and moved into her house. Cue all the bollocks on facebook from the OW about how she was now 'at one with her soulmate' and happier than she had ever been, yada yada.
Fast forward 8 months (to August 2017,) and the OW started contacting my friend asking her to let her DH move back in as she couldn't stand him being there! BA hahahahaha!!!
My friend said' he's not a puppy, you can't return him because it didn't work out!'
Upshot it, once she had got her 'man,' - the man she had been having an affair with for a YEAR before he left his wife, it soon started to go sour. When my friend did a bit of digging, it turned out she started to get a bit sick of him within 5-6 weeks of him moving in LOL!
He is still with her too, in her home, and she makes no secret of how she is fucked off with him. And he won't leave.
Needless to say, my friend is NOT having him back, and his own daughter and mother have disowned him. He has nothing and no-one, and the OW can't stand him LOL!
So hopefully the same will happen to your DH and his OW!