To answer your question, I think it's because cheaters are the ultimate in selfish. Everything becomes about them. It's THEIR happiness that they're concerned about, no one else's. They revel in the attention! The OW wants them desperately, the STBX wants them- everyone wants THEM! They feel empowered, in control. If you suddenly pull away, then a big part of what's feeding their ego gets pulled back with you. That's why sometimes a wayward will pursue a bit, to make sure they get you chasing again.
Additionally, you are a backup plan if things don't work with the OW. If you are acting like you won't be there, it's uncomfortable for him. He's left thinking that he might find himself stranded with nobody in his life.
