Again this is just in my experience, but it’s definitely an older generation (like Gen X and up) order. A lot of these people didn’t grow up with espresso coffee and they still drink garbage instant coffee + hot water and some milk at home. That’s what coffee is to them, so when they go out they get a flat white because it’s the closest thing to what they drink at home. If they want something fancier they’ll order a cappuccino and if they’re not a milk drinker they’ll get a long black. Because they drink coffee at home in the same way as they drink tea (add boiling water and sip over a very long period) they often ask for their flat white to be “extra hot”, which usually means absolutely scalding which of course completely ruins the milk but they don’t care as long as they can sit there and sip it for like 2 hours while chatting. The concept of coffee as something served to be enjoyed immediately and in a short amount of time is completely foreign to them.