I think there was such a selection pressure when we were nomadic hunter-gatherers. Agriculture is a very recent development anthropologically, so we were still in the mode of one-kid-per-pregnancy, though multiple might have been advantageous. It's possible we developed higher fertility due to that, but we didn't have enough time for that new pressure to kick in.

It’s been 5,000 years since the dawn of agriculture. If multiple births were fertility increasing they’d be, if not normal, at least common in some groups. You don’t even get 1 in 3. Single births are clearly not being found wanting by the Blind Idiot God.

Not necessarily. Mortality of mothers is higher in multiple births; maybe that's a counter-pressure (fewer children). Children do worse as well ([0], [1], [2]). Ladies are not built to carry more than one child, especially considering the size of our children, and agriculture has been such a blip on the evolutionary/anthropological timescale that we have not had time to evolve so significantly as to change our basic reproduction.

[0]: https://watermark.silverchair.com/151663.pdf?token=AQECAHi20...

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5541191/

[2] https://bmjopen.bmj.com/content/4/4/e004514

The size of the human brain makes this physically impossible.

Ok, explain triplets. https://en.wikipedia.org/wiki/Multiple_birth

Three and twelve are very different from each other. Even three is very rare and often comes with complications.

Minor correction: Advantageous to the children (conferring evolutionary advantage). Not to us.

I'm not sure I understand your distinction. From a gene-based perspective, the gene, whether in the child or in the parent, is the same. The selection pressure is on a gene level, not on a species or individual level.