That's a slight generalisation...
I said 'most'
Firstly, it depends on age group. If you have 4 kids in early 20s and aren't happy with who you're stuck with, you won't look happy. I had a kid (well she did) when I was 20 and I was energetic and happy, even if not always delighted to be in relationship.
I know a few people who had horror story exes and are now single, older and getting older by the day. They seem to have given up on life and being happy with someone.
I have other friends who have no kids, never have long term relationships and are happy as hell, love job and look great for age.
You cannot generalise. Someone could look down cos of job, family situation, whatever.
Do any of the people denigrating parenthood have kids? Don't give a cliche, cos unless you have one, chances are, you don't have a clue what it is like.
Their age plays def a role and yeah, every specific situation is different. In most cases it's prob due the stress, caused by raising them, full agendas, shorter sleeps, worrying about finances and so on. I have no clue? LOL, well being an observer myself, I'm no stranger to MILFs.