Why don't papers have their titles as a filename?
The filename is some sort of encoding that is compatible with the database in which they keep the papers, it might have a real world meaning or not. It is optimized for their own processing. I seriously doubt that they use a flat file system to store the documents. 

But in general, trying to use a paper's title as a file name won't work anyway, since some characters that might occur in a title aren't accepted in filenames on every operating system. Colon and slash, for example, are problematic. There is also the problem that a paper might need to go into the database during the acceptance/review process in which the title of the paper isn't finalized. 

In addition, while we like the titles of papers to be unique, that isn't always the case. So, some disambiguation needs to be done. 

What you have, actually, is a reference to the paper that represents its "identity". This is similar to a pointer or reference in a programming language. The thing referenced can change arbitrarily, but its identity doesn't. This help guarantee that they don't lose it.