In theory, you don't need college to learn much of anything. In practice, few people are going to correctly teach themselves discrete mathematics within a reasonable timeframe.
A graded evaluation is valuable, agreed. I submit that instructors, peers, and an instructional environment are also of value in aiding the learning process.
The degree is evidence of a graded evaluation, which is a proxy for education. Despite some propaganda to the contrary, it is not a zero-correlation proxy.
Also, I don't know about you, but I went to college for an education. Which I received.