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.