I'd assume in hunter-gatherer societies, children would not be that useful for hunting until they are fully grown, or in gathering until they've spent many years learning where to go to gather what. Also not useful in making tools, since those would take a lifetime to learn.
Whereas children are readily useful in traditional farming even at a younger age, because there are all sorts of places one can employ unskilled manual labor on a farm.
Hunter-gatherer societies had very low rates of demographic growth. And industrial societies are also seeing much lower population growth, because children are an enormous economic burden.
This gives an impetus to at least wait 3-4 years between pregnancies.
There is no such pressure on agrarian societies.
I imagine there was no(t much) fieldwork, crops were raised on the side, if hunting and gathering was indeed easier. What else would explain foregoing that?
Still today I sometimes dismissively assume that sowing seeds and just waiting is the simplest thing ever. Maybe neolithic people didn't need to process the crops either, instead feeding animals and eating them then, which is less efficient, energetically speaking.
People started growing crops out of necessity. In terms of food security, hunting-gathering was far easier. The average hunter also worked far fewer hours than the average farmer in antiquity.