This is definitely one of those "grass is always greener" moments...
My experience as the oldest of 3 kids was a major deciding factor in our son not having any siblings.
Well of course, I think that's what most people are trying to say, their own experience and why they chose what they chose (if in fact it was a choice, as clearly not every family size is determined by parental choice).
FWIW though, my husband is the 3rd of 4 kids and he always wanted at least 2 kids, never just 1. After our son was born he was quite satisfied with just the 2 cause he's such a handful

, but was willing to go for the 3rd anyway.
A big motivator for me is that my father is dead, so now it's just me and my mom. I feel guilty about not visiting her enough, I have to spend every holiday with her (not that I don't want to, but I mean, she has no one else), I feel like there's a lot of pressure on me even though my mom has never knowingly placed any pressure or guilt on me. When she had breast cancer and underwent surgery for that, I was left to think about what my life would be like if she didn't beat the cancer and I was left without either parent at 22yo and no one to share the past with. My husband (then fiance) was there with me in the hospital waiting room during her surgery, so I had support from him, but had she not become a survivor I would have felt very alone.
And I have a huge, huge extended family, with many relatives I have been super close to since birth. Some of my cousins are my best friends. In fact, one of those best friend cousins lost her mom to brain cancer the same time my mom had breast cancer, so we grew even closer while we went through similar battles together. After her mom died (and her dad, though living, divorced the mom and took his kids to court to get money from them after the mother passed) as great as it was to talk to me, it was her brother who could relate to all the stories about their mom, not me. Sure, I was around a lot as a kid and knew her mom very well, but I didn't live there, I didn't have the same childhood experiences as her, but her brother did.
I know having a sibling is no guarantee that won't happen, in fact my mom has several brothers and none of them would be a huge emotional help, but they are all in contact with each other and talk about their childhoods often. They couldn't be more different or have less in common than they do, and likely none of them would be friends if not related, but they all have a pretty good bond and care a lot about each other.
I will say that regarding splitting time between kids, that was a factor for me in going from 2-3 kids. Right now with 2 we can each take a kid, or having 2 on your own isn't that tough, but I know with 3 it will be harder. I figured that overtime that would get easier and the benefits would outweigh the negatives. If nothing else, I figure that even if my son and daughter don't end up being close, there's a chance that at least 1 of them will be close with this new baby, upping the odds they'll have someone around after my husband and I are gone.