There are probably a lot of reasons Adobe does what they do with Photoshop. Reading in a variety of places would suggest that they range from cost control, minimizing pirating and demand from millions of photographers that want efficient upgrade paths.
Adobe does provide a product to suit your "pay once" requirement. It is the incredibly inexpensive Photoshop Elements. It contains editing "power" beyond imagination only a few years ago. Unless you need a few capabilities unique to magazine printing and publishing there is little you can't do in Elements.