Because for most couples cuckolding is a they thing. They are looking to add some excitement to their sex life.
She wants the comfort of having her love with her sharing her pleasure and he wants to be there to witness it and share the emotional bonding they have. Not to mention both of them getting off on the kinkiness of it all.
Also there are safety concerns, at least starting out with a new lover. Until they get to know him and develop trust in that he isn't going to harm her, she is going to want hubby around.
None of the women I am fucking include their husbands. But I am in a group that vets new members. If a woman joins the group she will know what to expect. Members of the group are not only bed partners but good friends for the most part.I would imagine that without a system similar to this in place a woman would want the security of her husband present. Personally I have little interest in cucks or the cucks lifestyle. I prefer women that are looking for some discrete extramarital activity.
The most important part of extramarital sex is risk management. That means don't get caught and don't get a disease. relatively closed groups like the one I belong to provide the safest environment, and the lowest risk of contracting and STD.
I think it depends on your particular kink. I don't see any reason to include the cuck. If there is a safety concern he can sit in the living room and watch cable while I am banging you. If he wants to get off seeing you getting fucked then bring a video camera. I understand that for some cucks knowing that their wives are getting fucked and won't see the action until she gets home with the video heightens their final pleasure. It has been quite a while since I have had to deal with a cuck, I just don't see the point. I am 60 and I get get all the pussy I want on pretty much my terms, there is no reason for me to compromise to make some cuck happy. If he needs to watch she can either bring a video camera, or find someone else to fuck her.
I'd have no problem with my Wife going solo with a lover, however, as she would most likely want to include me for the extra pair of hands and mouth to lend to her pleasure, she would most likely look for someone OK with doing a threesome or more-some.