Some Apps cost money to run. Apps run on servers. Last time I checked, servers cost money to run. Like your car. Unless you're Fred Flintstone, running your car with your feet, your car costs you money. So while you're correct, a one-time payment might be swell for you, in some cases it would suck for the owner of the App. He/she would get a small amount of money from you, but after some time, the App would be running a deficit when you use it. It makes sense in some cases to offer a one-time price. I know I do, but at the same time, unless the App is some trivial crap that has no real CPU load, I need to ensure the App pays for itself.
What seems like a lot of overhead or money to you, is not necessarily the same experience for others. If you're racking up hundreds of dollars per month in Apps and IT but making thousands doing so, you're normal. My take on it is don't drive a Hummer unless you can afford the gas. Until you can, drive a Yaris and sip gas at 60 mpg... knowing you cannot off-road or run over senior citizens with nary a bump.