What's the point of PhD theses if nobody reads them?
For simplicity, I am going to let your premise that (virtually) nobody reads a PhD thesis stand, even though that's debatable and also a bit field-dependent. There are still a number of reasons for it to exist:


It's arguably more of a "writing to learn" task anyway. Students don't produce theses for the sake of the thesis, but to learn how to do research and write it up properly in a long, coherent book. Even if no single person outside the committee reads the thesis ever, it was still a good learning experience for the student.
There is a lot of history around the concept of a doctoral program requiring developing in new thesis (in the original meaning of the word), writing it down in a book (the dissertation), and defending this new thesis against the local learned community. Even though nowadays many fields don't communicate new research ideas through long books anymore, there is still enough historical appeal to the idea that few programs want to get out of it entirely. The entire process of writing and defending the dissertation also has some appeal as a significant milestone event, which nicely demarks the end of an era for the student - he is no longer a student, but a complete member of the academic community.
In many countries there is a legal angle to this. At least in Europe, a PhD program is usually legally defined to conclude with the production of a doctoral dissertation of some kind. 
In the age of "stapler theses" (which consist of a synopsis and a collection of previously published papers in verbatim), the entire affair is fairly low-cost anyway. My last students rarely spent longer than 2 or 3 months on the actual "thesis writing".