When market-based products (such as food or clothing or wine or gas (despite wine and gas being taxed, it is still governed by market values)) are sold, market competition drives prices down. That prices are high in those areas has to do with scarcity and cost of business, but is high despite things. Business want to make a profit but they also to a large extent want to undercut other businesses.
When you go to buy tax products, they inflate directly based on the government rate of inflation. There is literally nothing to drive the price of stamps down, so one day, a single stamp might be as much as $75. It is only reasonably cheap because the government deigns that it be so. If government wanted your money now, they could make it so prices of postage are absurd in comparison to other prices. They could even justify this as "preventing the post office from bankruptcy."
1. The stamps go to funding the US Postal Service. Thus, its no different than the stamps UPS puts on their packages.
They go to lining the pockets of politicians.
Those aren't stamps. When you go to UPS online, they tell you that they offer USPS stamps. Where are the UPS stamps? Where are the Fedex stamps? Oh ummm, those are illegal.
Just as restaurant do not make soft drinks but rather buy the tap from Coke or Pepsi, the UPS does not make stamps but sub-lets them from USPS. The difference is that I could technically buy a soda fountain, make an orange and a lime concentrate, and sell Orange SMASH! and XTREME Lime in my restaurant. I'd probably need a special soda license for liability of selling homemade soda.
But the USPS does the equivalent of saying nobody can make "soda". There aren't any homebrew stamps. They have actual security features to prevent makig much cheaper stamps.
2. They ARE a flat rate based on weight and distance. Have you never mailed anything bigger than a letter outside the US? Distance for parcels anywhere in the US is 1 rate with weight classes increasing the amount of stamps (price) you need. Once you go beyond the US borders, the price increases per stamp. You can't mail a letter from New York to England without more stamps.
3. What do you call UPS or Fedex when they take your money to mail a letter and put a label on it? That's a stamp with a barcode.
You're describing international prices. That's apples and oranges. National prices for letter delivery are a "flat" rate... that keeps climbing. Rather that charging a more reasonable distance and weight price (with regular letters delivered locally being free), we have a fakely cheap stamp system that is supposed to convince us that it's supporting the mail service. But bailouts are supporting the mail service.
Those barcode stamp things are printed using tools from the USPS. UPS and FedEx are private franchises of a very public goverment-run business. At any time, just as government could raise prices of stamps to outrageous levels, the Postal Service could simply require all private mail systems to be approved.