Why not?

I don't need the degree. I don't intent to pursue employment which will require it. And should I decide to continue at the graduate level, I already have a regionally accredited bachelor's degree and grad school. Thus, I have options for that path.

I like the structure and grades. Sure, I could I learn just as well using MOOCs, textbooks, and so on. However, the grade hovering over my head provides a touch of external motivation to stay focused. Plus, it's nice to interact with students from different places and with different interests.

So, basically, it provides me a cheap means to fulfill my curiosity.