I don't know much either, and this is all way before my time, but I'm going to guess that getting sound off the film (if it has it) has got to be one of the reasons.

The other being that just operating a suitable projector as intended is the simplest and most accurate way regarding timing compared to finding or writing software to handle scans. I'd think they'd want to do both.

The sound retrieval is a good point. I don't think of sound on old movies.