Is not being able to camp with you really a reason to walk away from these friendships? If so, it seems like they weren't terribly powerful or deep friendships anyway.
I disagree, and I think the reasons why are tied up in what I'd describe is the distinct "maleness" of these friendships. I don't mean to be sexist, but I haven't ever had this sort of friendship with a female, and I don't perceive it to be a common form of friendship among women, although I recognize I could be misjudging from the outside.
The friendships I'm mourning here are probably based on the ancient format of going to war together that taps into something deep within men: define a goal, gather a group of guys to accomplish it, and come hell or high water, you get it done together...My experience has been that the best friendships are about coming from a common background to engage in a particular challenge. It's not about having dinner together, or talking, or anything done under artificial lights. It's about doing hard stuff together. That sort of friendship is, in my experience, a deep one as well as pretty darn non-adaptive to the lives they've now chosen.
This is interesting. I do think friendships can be highly "gendered" in that we socialize men to invest in, expect from, and experience relationships differently than we do women (and there may be some underlying biological differences that impact this as well). I personally do not believe that there is anything that says things have to be this way, though, and I think by having a narrow definition of friendship (which is not to say it's not a meaningful one...just that opportunities for such friendships are in short supply right now), you are missing out. There is a whole wide wonderful world of different types of meaningful friendships that a person can have--why limit yourself? Of course, mourn the loss of those, but try to expand your definition of friendship at the same time and see what happens.
I can see a similar thing with DH, who mourns and laments the changes in his friendship, who believes that no-one will ever "get" him the way his best childhood friend did, or his army buddies did, and on the rare occasions when they still get together, I ask how they're doing, and the answer is generally "Dunno, we didn't talk about it" (and I think he's being forthcoming). And I think to myself, do you even know each other? You went through something alongside each other and that created a bond and that's important, it matters, but it's just one type of friendship. I have those kinds of friendships, too, the boarding school friends, the grad school friends, the building-a-business-together friends. And yes, those friendships are significant, but once the circumstances change, the friendships change, because they were built around those shared experiences more than anything else. Again, I am not minimizing the importance of those kinds of friendships, but they are only one type of friendship.
I had three kids in five-year-period, and yeah, it changed things (availability), but if my friends resented it, they kept it to themselves. The majority of my friends were childfree, and most of them remained that way, and our friendships endured. I can't think of a single friendship I lost, and if there was one, it was non-regrettable attrition. I think because our friendships are built around the person, not the activity, how we spent our time together was secondary. I cannot imagine ditching a friend because we can no longer do the same kinds of things together that we once did. We would just adapt. Like, when my friend got cancer, and couldn't go on long walks (which is what we usually did together), we changed what we did together. Or when another friend developed an issue with her eyes and couldn't tolerate bright lights, natural or otherwise, or another friend moved out-of-state for a job. We changed when and where and how we spend time together. Just like they adapted to my limitations for a few years when my kids were little.
ETA: I hope I don't sound unsympathetic. It sucks to have everything change through no fault of your own. You didn't ask for this.
AND, I think you are getting in your own way by holding onto (what I view as) a rather narrow view of friendship. So...as other have mentioned above, I recommend diversifying, like you would your investments.