I don’t know. Maybe because who each has played, relative to how good the opponents are on the national scale, are different. Maybe because in baseball, playing a certain team can widely vary in how good they are on that day depending on what pitchers they use. Which is the answer to the other point about why baseball plays series.
Winning 2 of 3 all year in baseball (against a quality schedule) gets you a regional at large bid. Winning 2 of 3 all year against the schedules UL basketball plays, gets you a bye in the conference tournament
I know both of you are smarter than this, and are likely just doing some trolling here.