A pretty flawed apples to oranges comparison. OS X is more similar to bios updates... you buy the hardware, the vendor makes money off it and gives away the software. With Microsoft, you're paying the OEM money for the hardware(with razor thin margins usually but that's another story), so Microsoft doesn't make bank on expensive hardware and upgrades to RAM/HDD/CPU that Apple makes.
Edit: Not to mention that 10.5 to 10.7 is not upgradeable, you have to buy and install 10.6 first, install the App Store and then install 10.7. Windows 7 may not be upgradeable directly from XP without a format, but atleast you don't have to but and upgrade to Vista first!