Even supposing children are raised communally by the tribe, female fertility rapidly declines post-30, and falls off a cliff post-35: https://www.britishfertilitysociety.org.uk/fei/at-what-age-d...

So you have very few fertile years left into which to fit multiple healthy children. 2.1 is needed to merely sustain a population (with all the medicine of industrialized society to keep them alive). For "wild" humans, that number would be considerably higher.