It seems that Southwest is still building their schedules manually. If it says no scheduled flights from BWI to HNL, but you can create one with a multi-city (effectively a one way) with BWI-OAK, OAK-HNL; or BWI-SJC, SJC-HNL, it means someone forgot to figure out the BWI-HNL schedule and store in their system.
The other airlines have been building the itinerary for your desired airports "on the fly" for many years. Rather than storing every possible A-B itinerary in advance, each individual flight is in a database, and when you say you want to go from A to B, the database will look up all possible flight combinations that will take you from A to B.
Back in the 1990's, getting an itinerary on a legacy carrier would take a few seconds, whereas getting an itinerary on Southwest was instant. That alone would suggest they had stored all the A-B itineraries in advance. They had some obscure city pairs that "didn't have scheduled flights" but in reality they did, but you have to buy two tickets (something like MHT to MAF).
Of all the improvements to technology in the last 25 years, Southwest still can't build intineraries on the fly, or operate red-eye flights? Ridiculous.