There are a multitude of reasons. In the case of my WS his was the classic conflict avoidance answer:
I, for one, do believe in many cases where there is 'lackluster' commitment on the part of the WS to R, the WS doesn't want to be the one to pull the plug.
My WH "did not want to be the bad guy anymore" - meaning that he hoped I would leave on my own so it was more "my decision". I do believe that he did not want me to hurt anymore, and he figured if I left and it was my choice, then there was less hurt for me AND big bonus for him - I was gone and he didn't have to feel bad. A "win-win" in his mind. I mean, how much talking to you need to do in a "mutual" breakup really? Sigh.
Things changed with my WH - but that wasn't for awhile. He admits now that for a long time he really did want me to leave of my own accord because he figured that would be better for me and better for him as I would be gone and things would look okay on the surface. He even admitted that he fantasized a little about playing the victim - like I left him and he was sad about it - and that the whole thing could some how end up allowing him to eat more attention-cake (the kind with the sympathy frosting) + he would still have his married AP to play around with + in his mind I would be happy in my new WH-free life, still "kinda pissed" but otherwise relatively unscathed - and heck, maybe we could be friends still somehow.
Yeah - that was his fantasy. The other part of that fantasy was that somehow the OBS, who was one of his best friends and work-mates, would somehow forgive him for fucking around with his wife for 2+ years right under his nose at work, that OBS and AP would divorce, and WH would slide right into OBS's place, taking the reigns in their social network (AP and OBS were quite the social butterflies in my WH's group of friends), while they all work work together happily ever after.
I mean - um - fantasy-land times 1000000.
I tell you all of this for three reasons. First, try to stop wondering what your WS's motives are. They are likely crazy-town in some form or another. Two, trust your gut - if you think your WS's words about why they are staying are bullshit, you are likely correct. Third, make a plan for yourself to get out - not because you are going to - but because it sure is nice to be able to IF you decide you need/want to later instead of scrambling like I did when I reached my limit and was financially stuck for a time.
You are the only person you are guaranteed to spend the rest of your life with. Act accordingly.
Constantly editing posts: usually due to sticky keys on my laptop or additional thoughts