Sounds like you've been going to the wrong mess dinners... The last few I've been to have been a lot of fun.
I can see why some prefer to wear a tux, but I can also see why some want to be able to wear their mess dress. If you want to put the military in your past, why be an associate member of the mess? There are lots of other social clubs you can join., some even have their own uniforms and accoutrements.
I think it's some of the pretensions and weird rituals that I object to (like having to ask permission to go to the bathroom like a school kid), but yeah, the last one I went to in the Ottawa city hall atrium was definitely not fun. Maybe a smaller mess dinner like a unit one or something would be different, as the last one I went to that I enjoyed was with the ship's wardroom right before deployment with about 25 people and was a blast. Navy ones are also weird, in that they keep doing them on Thursday and expect you to go, but don't think of giving a short day. Small thing, but annoys me as it's unecessarily petty.
Just from personal experience, but the few people I know that have or have expressed interest in wearing mess kit after retirement have all been people I didnt' like in uniform either, and their personality was their rank, so comes across more of a desperate attempt to stay relevant and have some kind of authority. If someone got the token respect due to their rank while in uniform, hard to maintain that pretence once they are retired, and people that earned the respect while in uniform will still get it regardless of what they are wearing.
I've learned a lot from retired members (some very senior ones), and still do, but their rank at retirement was more of a postmark fact when they were sharing their experiences and providing advice and mentoring. Again, maybe biased, but wearing your mess kit after you've retired has the same kind of energy as military spouses that want you to address them by their spouse's rank, and try and throw that (non-existent) weight around.
Is it an unfair generalization to assume that they are wankers for wearing a mess dress after retirement to a function? Probably. But the ones that are wankers and are wearing the retired mess dress will be that kind of bad example that overshadows others that maybe just can't be bothered to get a tux or something if their mess kit still fits. It's not like all cross fitters or vegans are unbearable, but there are unbearable cross fitters and vegans, and those are the ones you remember.