Simply put, because people are ignorant. That's why.
It's not so much to do with introvert vs extrovert either...
It's this whole "life timeline" that society has somehow imprinted into the heads of the masses...
When you're a teenager, people expect you to party. Apparently that's what all teenagers do.
(idk maybe some people get to skip that's stage of life completely then? )

Lmao
When you graduate HS, they expect you to know exactly what you want to do for the REST OF YOUR LIFE and pursue a degree and career.
Again...

WTF
Soon they're expecting babies and wedding bells...
Personally I think its just stupid, we're not sheep and we're not all clones. We do what we want when we want and how we want. We have a beautiful thing called OPTIONS!
In this day and age, women still get funny looks for not having "snag a hubby and push out some babies" on their to do list.
As if not wanting kids and being ok with being single is completely unnatural...
I'm a 30 y.o single mother and have been getting the wedding bells / siblings for my daughter 3rd degree for the past decade!

These have become quite "aggressive" in the past few years since dating my bf. Pfft
Now I just give them a disgusted look like I smelled something rancid and say " ugh! marriage is nOt our thing. We've got a good thing going, why would I wanna ruin that!?"...
After that they're usually too perplexed to reply and I walk off or change the topic.
People are stupid.