No matter what the rule is, there's a darned good chance that there's an exception to it someplace. Congress, at the Federal level, writes poor laws. What makes you think state legislatures would write sales and use tax laws that are any better? In fact, they're probably worse at writing this stuff.
So the laws are vague, incomplete, difficult to comprehend, and contradictory right from the start.
Add a quart of court decisions, both Federal and state, that render many of the laws either unconstitutional or modified in some meaningful way.
Add disorganized official bulletins and opinion letters that are hard to research.
Mix in auditors and support folks who don't have business training or even accounting degrees, and are out there giving "official" advice to taxpayers when they should probably be locked in a room someplace.
Stir in the fact that the business landscape keeps changing with new technologies and new business models and techniques. Many states still haven't figured out how to tax downloaded software!
Add a dollop of the lack of training, beyond the mechanics of filling out forms, offered by the states. Not that I'm really complaining. Their failure on this one keeps me working.
Add another dollop of the lack of education that lawyers and accountants receive on this topic, thereby creating an entire industry of income tax professionals who don't know what they don't know about sales and use tax, but still give advice. Again, not really complaining.
Sprinkle on the spice that every state does it completely differently; that what's taxable and what's not taxable varies almost completely whenever you cross a state line.
And, to complete the baking metaphor, mix it all together and you've got a messy, gooey cake. But it's tasty for sales and use tax professionals, and me, because we make money off the confusion. Hooray!
Sales Tax Guy