Because then the issue wouldn't come up. You also wouldn't have screaming babies at all. Parents with newborns need to sacrifice some things, like going to nice restaurants with their infants. Your child is not my burden and I don't want to hear it screaming while I'm dining. Common courtesy would be keeping the infant at home.
I dined out plenty with my infants and if they screamed I left. But there were plenty of times, especially when they were newsborns and sleeping, that they didn't make a peep.
Parents make plenty of sacrifices and once you have a child, you'll realize how selfish and judgemental people are toward those with children.
If we used common courtesy to keep "burdens" away from us at restaurants half the people would need to leave because of their flipping cell phones. Take half of those left and remove them because wear too much perfume and it bothers me. Take half of those left and removed them because they've imbibed too much, are rude and loud and it bothers me. and so on, and so on.