Why are URLs in XML namespaces?

Question

Take this line as just one example:

Answer 1

The core of the problem is that namespaces have to be URIs. There is no good reason for such a thing since all it needs to be is a unique identifier to give context for the defined elements.

Since they have to be URIs, all possibilities are senseless, because a scheme is needed (e.g. http) even though a namespace has nothing to do with such a thing.

So to answer the question of why: like all bad ideas, someone thought it was a good idea at the time.