That's usually the case for most Humanities disciplines (history, languages, literature etc), because their overheads are really low and there is an abundance of teaching professionals compared to actual audiences.
The opposite is true for "hard" sciences like chemistry, engineering or CS, where expensive laboratory equipment is necessary and where teaching resources are scarce (due to higher private-sector demand). There, courses are expensive, demanding and small in number, so they are usually inaccessible to the layman.
That's not really true. It's not challenging to find CS courses online, and the year I spent in a CS graduate program (through Colorado State), the cost per course unit for CS was the same as any humanities course.
I believe toyg already covered the split between humanities and science/engineering, but I'd also like to cover the Continuing Education departments themselves.
The course offerings are generally constrained, and the courses themselves are often lightweight versions of the university course -- assuming that the subject you want is covered at all. The facilities for continuing education courses are also often less robust.
Take CCA in San Francisco. This is a highly considered arts college, and they offer a small number of introductory extension classes:
You'll have to drill down on individual categories to explore the full breadth and depth.
Continuing Education offerings are rarely a substitute for the quality of courses they offer to a matriculating student.
Given how often educational institutions have raised their tuition in recent years, I'd love to see them institute a program where they charge a (possibly large) fee to allow non-matriculating adults to audit a course, without limitation or requirement, and use those fees to subsidize enrolled student tuition.
However, this may be a shift too far in the "trade school" direction for their comfort.