I wonder how much of it has to do with the fact that birth control offers us a choice in the matter. Pregnancy used to be something You Just Did, because, well, most people enjoy having sex, and pregnancy is the intended consequence of unprotected sex. It's scary, it's stressful, it's risky, and it's sometimes even fatal... but it's "part of life". It's how we ended up here, after all.
Now we have a choice. Which is GREAT, amazing, even. Absolutely necessary. I'll never argue that anyone's right to choose if they want to be a parent is a bad thing.
I wonder if social media is also contributing, with its polarizing nature. It's rare you see any opinions or even anecdotes about pregnancy being "fine" or "whatever". It's always a big deal, and it's always a big deal (horrible) or a big deal (wonderful). There's not a lot of in between. The negative end of the spectrum scares people, while the positive end of the spectrum is met with skepticism since it's too-often (for me at least) correlated with right-wing religious people trying to push women to be what're effectively baby factories, so the "it's the most wonderful experience ever!" just comes off as disingenuous and manipulative.
Combined with the fact that women's healthcare is rife with challenges, and women's pain is often dismissed (not looking to start any "gender wars" arguments; just stating things that can be backed up https://pmc.ncbi.nlm.nih.gov/articles/PMC10937548/), the prospect of becoming pregnant is a scary one, when you have the option to not do that.
"What if it's nine months of constant nausea, being bedbound, gaining weight I'll never be able to lose? My body will change, in many ways irreversibly. Will my partner still find me attractive? Will they support me during? Will they support me after? Will I feel supported? Will I feel resentment that they'll be able to enjoy things like alcohol while I'll have to stay completely sober for a very long time? Will I feel resentment if the majority of childcare falls on me, which is statistically actually quite likely? What about my career? Will I regret it? What if my child is born with a disability? What if I'm bad at being a mom? What if I hate it? What if I'm trapped in a life I don't want? What if there's no way out?"
Of course, the above thought process could also be one with thoughts like "what if it's wonderful?" but it's a lot easier to worry about the negative, and put weight on the negative, than it is to fantasize and put weight on the positive what-ifs.
This is just my perspective, though. Some people love being pregnant and find parenthood really wonderful (or so they tell me about those things), and I love that for them. Truly. I'm just not sure if I have it in me to ever find out what it would be like for me, and I know I'm not alone. Fun Auntie is a better role for me I think. But maybe I would feel differently if pregnancy were easier and a less destructive process on the female body, and there was more social support for parents.