Does a Ph.D. dissertation usually have an ISBN?

Some institutions publish each PHD thesis as a book, and then they assign ISBN to each. E.g., theses done at CWI, Amsterdam. To my knowledge, this is quite uncommon throughout the world, but does exist in certain places.

I suspect it largely depends on whether the PhD is expected to be printed as a book. In some countries (like the Netherlands) it is. Many printing services offer ISBN registration as a freebee. This is likely how most dissertations end up with an ISBN (I know mine has one because of this).

Other than this, I doubt many graduates go through the trouble of registering manually for a PhD. I don't know of any place that explicitly requires it. So in short, it's not expected, but it's not that unusual for a thesis to have one.

No, it would be unusual for a thesis to have an ISBN. It would be an unnecessary expense.

Edit: The Astrophysics Data System says my dissertation has an ISBN. However, the ISBN provided cannot be used to find my dissertation.