My take as a former professional classical musician is to say that your idea of PR boosting ticket sales is not valid for (at least most) classical music. That's not to say you're incorrect, just to add a different (hopefully slightly interesting) perspective from another genre with a very different audience.
One reason for this is that classical music is focussed much more on the music than the musician, when compared to "popular music" (what's the best term for non-classical music that doesn't sound patronising?). Sure, Yo-Yo Ma or the London Philharmonic can attract people based on who they are, but even then a lot of decisions to see them or buy their recordings will be based on what they are playing. If you're a Katy Perry fan, chances are slim that you really want to see her live, look up a concert, then say "ah, but I'm not a fan of those songs, I'll give it a miss".
Another reason is probably range of repertoire - again this comes down to caring more about the music, not having the piece tied directly to the person/group who always performs it. If you love Francois Couperin's music, you're going to find it a lot easier to buy recordings than find performances. I suppose the same is true for indie performers, but I think a classical listener is more likely to find a wider range of classical music that's hard to see live than the equivilent situation for indie music.
Probably plenty more reasons too, but not too much off the top of my head. Oh, and demand I guess. A classical music concert will typically either perform once, or perform a short run, or perform a few times in different cities (or countries). That's the case even for the very biggest performers. A big pop performer, on the other hand, can do a much longer tour, and can sell far more tickets for each.
As to recordings, I think classical recordings have a longer shelf life - sure, some people still buy music from the 80s, or whenever, but much less than around when it was released. Whereas classical music, all the music is so dated (not in a negative way, just literally) that who cares if it was recorded a month ago or thirty years ago, if it was originally written centuries ago. On top of that, I suspect classical music listeners are less likely to pirate music. Possibly that's a generic snobbish view, but I think it's accurate.