I've been in this situation for 6 years.
My STBX (hopefully) is also kind and helpful to everyone outside the house.
Why won't he leave? Because he needs me for childcare (he's said that) and it's comfortable to have a wife doing everything for you. Who wouldn't like to have live in nanny and housekeeper, laundry maid, cook and cleaner rolled into one who you don't even need to tell what to do, they just get in with it! He also wants to keep his image to the outside world of Mr Kind, Helpful Family Man.
My advice is to not do what I did and take ages before I understood all this. Accept that the Why isn't important and bite the bullet and go to a lawyer. Just do it. Do not faff around with mediation as he's manipulative and all that will happen is he charms the mediator with his image. The psychological fallout from that cost me a year of my life - I wish I was joking. The mediator ended up basically being involved in his abuse of me.
Don't talk to him about it. Just use 100% of your energy to get legal advice and start the paperwork. I don't know the exact process, but just get it started.
Why he's doing this is ultimately because he considers you lesser than him so thinks he can. You're not going to change that, so don't even bother engaging. Just get on with the divorce.
And the divorce is going to be tricky at times. I can absolutely guarantee you, however, that it's better than tiptoeing around him, waiting for him to move out, or engage properly while you're manipulated into giving away the best years of your life to someone who values you less than he would an actual nanny of cleaner (because he'd put on his good face for her!!). Never forget that life without him is worth the pain and struggle involved in divorce.
What sh*threads men like this are.