Don't have to fight the airline for compensation for a flight that was cancelled if the airline doesn't have your money yet.
This sounds insane until you realize that this is essentially how hotels work. You pay a deposit, and the rest isn't owed until you are physically in the hotel.
Or everything else. Trains, taxis, restaurants, everything.
How many institutions do you know where you can't buy a ticket at the door, and instead have to prepay?
Yes, there are a few services you post-pay like restaurants and taxis but you probably don't want to make a practice of running out on your bill.
Rental cars are something of an outlier in that, in the US, you can mostly book with major brands and just not show up with no penalty.
Sure they do. Maybe not literally at the door because you have to go through security first. But they'll sell you one at the ticket counter if there are seats available.
Right, that's what the deposit is for, and that's how airlines would presumably do that.