They treat it like a drug and lose all emotional regulation. I don't believe all screen time is bad, but it is something you have to teach them to regulate and 3 year olds and younger are just bad at regulating emotion in general. Teaching them to do this is just part of parenting. One of the most important things we can teach our kids is that it is okay to be bored. In fact it is great to be bored sometimes.
On the other hand, being a parent is hard and keeping your sanity is important in order to be a good parent. So if it helps you be a better parent all other times, it could be worth it.
The issue is when screens are used to in place of parenting. Parents using it as a way to fuel their own screen addiction.
On the other hand, for me airplanes are a special case and all rules go out the window to help keep the kid calm.