Why can't parents teach their child how to behave on a public transpo?

Can some parents teach their kids how to behave on public transportation, especially on a plane? Last night, I was on a 10 PM flight with a child and their parent. The child cried throughout the flight, and the parent did nothing but allow the child to throw tantrums. Some people on the plane showed irritation and tried to figure out why the child kept crying. I assume everyone was tired and just wanted to rest. I understand that such behavior is inevitable since they’re kids, and all we can do is be patient. However, I hope parents can talk to their children beforehand about the importance of staying quiet, respecting others' need for rest, and finding ways to cope with discomfort.