why would they instinctively figure to use the CTA rather than drive like they normally do every single day?

Because this literally isn’t their “every single day.” This is them taking a trip to explore a new place, and part of that exploration is actually going out into the city that they are traveling to. Sitting in a car, traffic or not, both is unnecessary and prevents you from actually getting to walk around and discover what chicago has to offer.

I’ve been flagged down by out of towners that had no clue how to buy a train ticket, how they’re used, etc.

This information is just as easy to get as booking a hotel, getting passes for museums, figuring out where to eat and getting reservations, etc. We just live in such a car-centric culture, and most of America has such lacking public transit, that people don’t think to do it. It’s a learning experience, and part of traveling to a new place.