Because he's not meeting either of her physical or emotional needs, they're basically separated but just live together.
I don't think they're even a couple especially because of his attitude so it doesn't feel like cheating, no point for her to use the mental shackles of marriage.
He doesn't even want to discuss it, even when she's been hurting for so long. For the sake of her happiness, I hope she leaves him asap`.
Unless he has a medical condition he's too ashamed of to discuss to redeem himself or something...