Yes it is "strange" that the U.S. Postal Service does not offer subscriptions to its stamps. I have no idea why.
Most USPS customers are domestic, within the US. A subscription is a contract. Almost all contracts are governed by US state laws, not uniform federal law. In the past, subscription contracts have been subject to abuse by private enterprises, some of which have made it difficult for consumers to exit those contracts and stop having their credit cards charged. For example, with some, it's impossible to terminate the subscription using any online facility, you must call a customer service representative on the phone and those people are trained to try to retain you, often forcing consumers into uncomfortable arguments. In response many, but not all, US states have enacted laws regulating how subscription contracts work, usually making it easier for consumers to exit. The result is a mosaic of different laws across the country. Given this, it would not surprise me that the USPS doesn't want to deal with 50-state compliance issues in this area.