People who support businesses and large amounts of users who believe they know enough to do what they want, understand UAC and what it does for them, for everyone else, it's a soft protection, it's not supposed to be the "be all and end all" of protection, it's there to prevent silly mistakes and stop simple nasty things from doing basic negative tasks which can be easily done in earlier version.
There always has to be a balance between security and usablity, most don't seem to understand this and truly believe that their idea works for everyone and that there is a way to have highly secure systems with little to no impact on the user.
It's simply not true, and if it were, I wouldn't have to say this.
Unfortunately for Microsoft, it is easier for them to put in some annoying (and I agree, they are annoying) prompts, than to deal with large amounts of systems with millions of variations of issues.
I'm not keen to argue about this, but this discussion has been going on for over a decade and no one generally listens to anyone so there is no point
For the TL;DR:
There is no perfect solution, one day we will get close, but there always has to be some sort of prevention system in place, and there is no system smart enough to make decisions without some form of user input.
To kind of stay on topic, UAC isn't the best either, but it's a step forward, and I'm of the opinion that it is better and less annoying than Vista's version (which I liked as well)