I would attribute Apple's decision not to divorce the iPad from a PC to their desire to assert some presence onto the PC ecosystem. They don't want to be be JUST your tablet OS, they want to be on the desktop, by forcing installation of iTunes onto Windows machines as well.
This way, both products reenforce each other, and they can get increased market share on devices and onto Windows machines (with an activated credit card so you can purchase music directly on the PC).
Apple is ultra-competitive and they do not want to cede any niche of the market that they have managed to gain. Even though the experience of using iTunes, especially on Windows, is horrendous (for all of the reasons already stated in this thread). That's that only logical explanation for maintaining this extra complexity by a company that is otherwise brilliant at simplifying the user experience.
BTW, I do think Google has approached this with a much better vision - treating an Android device as a first-class network citizen. Google does sell content (app market as well as enable 3rd party music stores), most Android apps integrate some form of cloud storage for your data (which don't have to be managed by Google) - if you switch your device, you'll not likely loose anything important to you.
Updates also work very well over the WiFi network - no need to use your low-bandwidth data plan.
Overall, I don't see this as a technical hurdle for Apple - they just have a conflict of interest in locking up users vs. giving them the best user experience.