Things get loose. Yes, you can get ripped and try to lose weight, but your arms won't look like they did when you were 22 and that is perfectly OK.
This is especially true if your abdomen has grown a lot while pregnant, which has led people to reclaim the beauty of their post-pregnancy stretch marks and belly rolls — which I love!
I dont know why this happens, but it's something more than just gaining weight or getting wrinkles. No doubt it has to do with collagen and muscle mass and hormones, like everything else.
You can get them lifted or enhanced with plastic surgery, and if that's meaningful to you, do it.
It has to do with hormones, skin elasticity, changes in adipose tissue and connective tissues, but the most important thing to remember is that it's natural to not look as "cut" as we get older.