In carrier, they are able to sleep and you walking around makes them sleep. They also can wait without milk for an hour or so. They don't eat constantly.
In car, they are in the back, so if they cry, you have to stop, exit car, deal with them, enter again and so on. And driving while baby cries in the back is annoying. Public transport and carrier is not that much worst, assuming you are not carrying too many things.
More importantly, I compared difficulty of toddler and baby. Toddler in car can be noisy too.
I mean, I had both kids and car. I also used public transport. I also breasted - personally, I am woman.
Calming crying kid on carrier is easier then while driving.
Breastfeeding in car is not all that comfortable and you cant drive while breastfeeding. Typically it is not that difficult to find reasonably private place within time limit since kid cries constantly. You are going to spend time somewhere, park or friend or shopping center. That somewhere has usually good solutions, otherwise you would not be spending time there.
