There are two problems with letting just anybody upload images: 1) Copyright law is very complex. Most people don't understand it. 2) Most people *aren't aware* that they don't understand it.
By letting everyone upload images to Wikipedia, we're telling them that we trust them to understand what is and is not allowed. Then, we turn around and bite them by deleting their images when, inevitably, they make mistakes. It confuses people no end to be told that they both are and aren't able to upload images correctly.
I see this confusion every day, in the questions on my talkpage, or on OrphanBot's user and talkpages, or on the image-related pages in Wikipedia:-space. I haven't had OrphanBot go through the 35,000 talk pages that it's left messages on, but I expect that if I did, I'd find hundreds more questions, unanswered because nobody ever saw them.
I also see this in the form of users floundering around, trying out different templates to see which one will get the bot off their back, or using clearly-incorrect templates based on a misunderstanding of copyright, or making up template-like statements because they don't understand what a template is, or providing "fair use rationales" that amount to little more than a statement of "I once heard these words in a copyright context".
I don't have the time to deal with this confusion by explaining Wikipedia's policies to everyone, and most uploaders don't stick around long enough to learn. The best I can do is respond to the occasional user who finds their way to my talkpage.
Much of the image deletion policy is based around the fact that there are maybe a dozen people on Wikipedia who understand the image use policy well enough to enforce it, and are willing to take the time to do so. At the same time, over two thousand new images are uploaded each day, adding to the 553,000 images already on Wikipedia.
By making image uploading a privilege to be earned rather than a right conferred by registering an account, we can relax the policy and deal with uploaders individually, rather than automated notification of problems and nearly-automated deletion of problematic images.