Why aren't there CS degrees with only 3rd and 4th year CS subjects?
I'm sure there are some incoming students with enough CS and programming background and a willingness to learn on their own so that this would work out.
By offering a pathway to get the same degree with less courses, they'd lose money. Many students have wished they could somehow escape from all kinds of bullshit courses, but universities have never allowed them.
In my experience, public highschools are better about this. In highschool, I saved a lot of time by studying on my own and testing out of courses. It was great.
Just to pick one example, I would bet that most self-taught programmers don't understand recursion which is taught in university intro CS. There's going to be a lot of material in the first two years that is new to people.
A four year accredited program ensures students take four years worth of course credits. If programs reduced requirements merely for knowledge rather than demonstration of an ability to act on that knowledge, it would likely reduce the value of accreditation. My first guess, anyways.
Courses for 3rd/4th year students tend to build upon knowledge from the first two years. Especially in fundamental subjects like mathematics and formal methods, I would doubt that this would work for students coming into year 3 without any previous formal CS education. These students might know some programming or other basics, but - as you certainly know - CS is much more than programming.
"Computer science is no more about computers than astronomy is about telescopes."
Some places will give "credit for prior learning" or "recognition of prior learning". This is mainly for people who have worked in the field before and requires evidence from say an employer.
A two-year MS in CS is precisely that.
Most American universities allow precocious high schoolers to graduate
in three years, but unless tuition money is a concern, a fourth year to
soak it all in is worth it. Your working life is long -- one year is nothing.