Why do fast food cashiers often mishear your order?

Because most people who work fast food are between the ages of 17 and 23. And usually, these people are morons. They are working fast food for a reason. I once ordered 2 Sausage & Egg McMuffin and instead of received 4 McGriddles.. 

It Is quite Noisy behind them in the kitchen and in front of them in the eating area.

Food service, particularly fast food service is now dominated by minorities-probably because the entitled snowflakes refuse to work for minimum wage. 

And with that, many are non English as a first language speakers. So accents not theirs may confuse them, or perhaps they speak little English to begin with. 

Years ago, I ordered food at a Taco Bell in a local mall. The cashier spoke No English. I paid with a $10 bill. Not only did I get the wrong food, I got change of $100 bill, and could not convince the cashier she was wrong. I spoke to the manager later in the day and refunded the money. She told me that she had no on else to put on the register since everyone else had called in sick. 

There is a McDonald's in Penn Station in NYC where you Cannot order a meal in English-No One speaks it, not even the manager. You have to order by pointing to a picture. 

Clearly, for someone like you who is superior to those lowly dumb people who serve you, it's understandable that you have such short patience.

Everyone is different. Personally I have short patience for people who think they're better and smarter and more worthy than everyone else. Talk about ignorance. When you think you're the smartest person in every room, you do nothing but grow more ignorant every day because you never learn anything.

You've constructed a pedestal to stand on and look down at other people. I've worked with homeless people and with corporate executives and everything in between. The fact is that rich powerful executives as a group are no smarter than someone in an entry-level minimum wage job.

And the people who run around crowing about how much smarter they are will be the ones that are clueless.

People rarely order water.  Next time say 'instead of soda can I have a cup of water instead?'  It puts the drink automatically in the right category. 

because of the mask. You should try it for a few weeks. you might not be any better.