Not sure how they do it now, but the old “shuttle trips” were easy flying. 4-5 legs a day but you kept the plane, same gates, same freqs, no bag or pax issues, never early never late (unless IROPs in which case a turn would cancel anyway) good BOS layovers and, at the time, free food and magazines (for the overnight).

It’s about much more than legs per day. I’d rather do 4-5 legs a day keeping the jet than fewer legs but changing planes every flight.