In certain less developed parts of the world, having kids means having extra laborers to support the family, and not having them means that you will starve when you are too old to work. In certain parts of the developed world, there is much more support for parents than there is in the US (such as publicly supported daycare), and higher education is free or inexpensive. So yes, the level of sacrifice depends on the society one lives in.
As I said before, whether one’s choice is selfish depends on one’s specific motives. I’m sure we can think of cases where not having children would not be a selfish choice for a specific individual; I know people I would put into this category. I also know others who do not. And “cannot” is one of the most misused words in the English language. People are often lying to themselves or others when they say they cannot do something.
Bringing a person into the world accidentally (I assume this what you mean by “without wanting to”, not people who are somehow forced by others to have children), without having the means or maturity (and who gets to set the standards for those things, I wonder) is not necessarily selfish, though it may be thoughtless. There’s a difference between thoughtlessness and selfishness. Quite a few of us came into the world unexpectedly, born to parents who weren’t ready (is any first-time parent really ready?), and life goes on. Anyway, I would expect that older parents are more likely to put some thought into whether they want children, and have higher levels of means and maturity.