Urgh, I'm really late in replying, but I think a lot of it might be that Snape is almost obsessed with maintaining his dignity, so it's hard to imagine Snape making himself vulnerable in that way and giving Harry the upper hand. And in light of the stigma of a teacher pursuing a student, Snape would be risking even more by laying himself bare.

But I would love to read a well written fic with this scenario.