Laugh if you will, but I've found sketching out mockups in PowerPoint to work surprisingly well and about as fast (or faster) than paper since I don't have to redraw the whole thing after too many changes are made. And to your point:
You can’t click a Photoshop mockup. This is probably the number one reason we skip static mockups. They aren’t real. Paper isn’t real either, but paper doesn’t have that expectation. A Photoshop mockup is on your screen. If it’s on your screen it should work. You can’t pull down menus in a Photoshop mockup, you can’t enter text into a field in a Photoshop mockup, you can’t click a link in a Photoshop mockup. HTML/CSS, on the other hand, is the real experience.
You can hyperlink in PowerPoint to other slides, so you can mockup the workflow a bit too. I agree that being able to click and move around a mockup is incredibly important.