I’d love an answer other than “because that’s just the way it is”. I understand it’s tradition. I’m curious why.

Almost every other major sport allows constant subs to keep players fresh except baseball which is understandable.

There are plenty of sports that test endurance like running, cycling, etc. I get that it needs to be a component but wouldn’t the watchable product be so much better if teams could platoon different formations? It would not only lead to fresher players and more excitement but also the ability to implement strategies and personnel groupings that are fun to watch.