a. other airlines will charge for baggage

b. other airlines will advertise prices that don't exist: it'll be the base price for the seat, but all (remaining) seats will be of the upcharged variety, so you can't pay the base price.

When you make the adjustments to compare apples-to-apples prices, SWA's get a bit closer. (But I do think SWA does tend to not be the cheapest, unless they're having a sale.)

I actually prefer the per-flight¹ pricing, too. That's what I'm buying: two flights.

¹IIRC, the pricing is per-flight, not per-leg/segment. (But it isn't, as you say, round-trip.)