That’s nice for you, but that path isn’t for everyone for all sorts of reason.
Some just don’t want to be married or have kids young, that just doesn’t sound like fun to them and that’s fine. They would resent settling down that young and as a result wouldn’t make great partners/parents at that time.
And others might want to but aren’t sure if they’ve met the right person, or if they’re financially/emotionally in a good place to start a family.
Due to my childhood I was aware my picker was off, so I was always quite cautious and knew I had a lot of work to do before I trusted myself to choose a decent man.
I mentioned upthread my friend who married/had kids young and is also flying in her career and has a great social life. However equally I know friends who settled down young and bitterly regret it - usually due to the kind of man they eventually realised they were with. One was divorced with 2 kids by age 26 after extreme DV, another divorced in mid 30s after a decade of her having to do and be everything while he did increasingly little and she ended up severely unwell due to stress. They both wish they had waited until they were older to settle down with someone.
A lot of it is luck too as far as finding a (good) spouse goes. I have a childhood friend who at age 21 went home with her first “one night stand” who ended up being her forever guy. She got pregnant 6 months later and is still married almost two decades later. Moving at that pace so young often ends up badly but luckily he was a good egg.
Someone else might choose their partner more carefully and vet them for years only to end up with a closeted narcissist cheat or the man who walks out when his wife is pregnant. We see it all the time on here.