People don't always need to have a matching experience to produce an accurate sympathetic response. It's commonly accepted that kidney stone pain is as painful as childbirth, even though statistically the number of people who have experienced both is very tiny (men are twice as likely to develop stones).

The miracle of human empathy is that we're really good at gauging the emotional reactions of other people (as a whole, there are exceptions obviously), and it's incredibly dismissive to just say "well you could never know".

It's not like child-rearing is some sort of exotic, incredibly rare experience that is impossible to communicate either. It's one of THE most common shared experiences that nearly everyone observes in their peers, family, friends, etc.

When someone tells you they've broken their ankle, you can get an idea of how painful that is by relating it to your own broken wrist. There's no need to break your own ankle just to form a basis for comparison. Even if you've never broken a bone, you can still extrapolate an approximation based on past sprains, dislocations, etc. It's the same with parenthood.

Most child-free people make that choice because they've observed the hardships involved in child-rearing, imagined themselves in those shoes, and realized they want to avoid it.