UV bulbs are expensive (and invisible unless you get to poke around a buildings mechanical room.), they need to be replaced typically ~annually (they won't be). If they get dust on them, they don't work as well. Etc. etc.

And here's the issue: To actually deal with high concentration infectious aerosols you need to get that air out of the room ASAP. (i.e. negative pressure rooms) If you can sterilize the return air that's great, but frankly I suspect dilution and air movement is going to be the dominant effect in reducing infection chains.