Childfree by choice here. I don't belive the below, but these are the kinds of attitudes I have come across.
I think it is because people assume you're childfree because you want more money, more time for yourself, more traveling, less stress etc than you would be able to have with children. So that feels selfish. You're not willing to give up your time, money etc to look after others.
However, as people have pointed out here, having children also entails selfish decisions (wanting a cute baby, cuddles, family time etc). It is just these are so "default" they aren't even critiqued, dismantled or acknowledged.
People think: "why don't you want children? That's so strange". Then someone goes "it's because they're selfish".
Another conversation: "why do you want children?" Then someone goes "well it's natural. That's the reason we're here, to procreate and pass on our genes. It's the whole point of being alive".
Having children = natural, default, and not in need of critique and explanation
Not having children = deviant, and a sign of personal deficiency: Either you're not able to have them or psychologically devious/defunct in some way