Your question is about why it’s rude, not why people have kids when they’re hard up, so I’m going to address that. Not everyone finds social nuances easy, and for a lot of people it can be hard to understand why saying something ‘true’ or direct might be ‘rude’.
It’s about the emotional function of conversation.
The most likely effect of telling someone they’ve made decisions you wouldn’t have made in their position, is that they will feel isolated and ashamed. When someone is already having a hard time, it’s not kind or helpful to rub their nose in it by pointing out the ways you feel they have exercised poor judgement.
It would be rude to sit at the bedside of someone dying of lung cancer and say, ‘well, that’s what you get for smoking.’ This might be logical, but is not going to be helpful or comforting. And it won’t be news to them. When someone is already having a hard time, it is unkind to say things that will likely increase their suffering. It’s a case of if you don’t have something nice to say, don’t say anything at all.
Also, as a pp pointed out, it is hurtful to say that someone should not have been born. It is hurtful to people who grew up poor to hear that their very existence would have been best avoided. Sure, that may not be what you mean by saying ‘don’t have kids when you can’t afford them’, but once the kids are there, commenting on whether or not they ought to have been brought into the world is insensitive at best.
We all go through life and make various decisions that make our lives harder. We know the ways our own decisions and behaviour have contributed to our struggles.
I also wonder whether, as someone who does not feel in a good position to have children, some of your own feelings of grief and loss colour your reaction here. It can seem so unfair that people go ahead and have children, when you don’t ‘get’ to have them - because of your own circumstances beyond your control, and the choices you make in light of them. It can feel unfair that other people
feel able to forge ahead regardless of practicalities, and you don’t.
These are such emotive topics. And I am sorry for the r struggles you are dealing with.