Do people have kids just because they feel like it?

I have two SIL, who both already have 3 kids each. They're both stay at home mothers, never lifted a finger in their lives, their husband is their only source of income-- both has fancy luxury cars and both has a closet full of Chanel and LV purses. One of them lives with my husband and I with her her husband and 3 kids, they don't pay any rent whatsoever (she's 31) and now she's pregnant with baby #4. Why? Because she was jealous of all the attention that I was getting, as I'm pregnant for the first time with our 1st baby. Now the other SIL, they live separately, but already have 4 kids as well. She just announced that she's also pregnant, and it's because "she missed the smell of a newborn baby." I'm 35 weeks pregnant and I'm still/have been working full time, and I also don't intend on being a stay at he mother. I just don't understand how these women can just bring a child into this world for such trivial reasons, such as "I like the way I'm being treated when I'm pregnant." I personally wouldn't bring another child into this world unless husband and I are financially and personally stable to do so. My BIL's must be extremely stressing right now. Lol thanks for listening to me rant..