Why people don’t go for treatment for the 2nd kid

I have seen if anyone have already 1 kid which was happened naturally, then for 2nd kid even if they are unable to conceive for long time, still they don’t prefer any treatment to conceive. They want to go for natural try only if at all that happen. And if not the prefer to stay happy with 1 kid. What is the reason for this? They don’t want to take any risk on their health? Or any other things? Can anyone please suggest? Thanks.