There is no feedback whether they are talking too quietly or too loudly.
My phone will pick up my voice if I whisper as well as if I yell, and the person on the other end will be able to hear either just fine. People just have a hunk of metal that they hold up to the side of their face (not really in front of their mouth), so they think they need to compensate.
If the TV ads for phones showed people whispering into their phone and advertising how good the microphones and noise canceling is, then we might not have this problem, but they don't.
Beyond this, yes it’s rude to talk at length on a plane to other people on a long haul flight, for example. I mean I get it but flying is by default a stressful experience as is.