I know tickets are expensive but I don't understand why people don't buy tickets for their babies.

In the early 2000's when I still didn't make much money and was a part-time student, I always bought extra tickets for our small kids. My daughter had a ticket on her name before she was even born. We would take our car seats with them. And they just loved to fly - people would compliment us at the end of the flights on how well they behaved.

One time we were flying on an Airbus with either TAM or GOL (same size as the 737, forgot which one) for a domestic flight in Brazil out of GRU, and the flight attendants were saying that we couldn't have our car seat in the airplane (by the window). I told them I had been flying with a car seat for a long time without problems in the US and Brazil. They called the captain and the captain told me he read the manual and didn't find anything against it, so I was allowed to have the car seat with my son at the time.