How about soccer and hockey? They both have to have mad skills. Just for reference, if the US Mens National Team had a critical soccer match tomorrow. All but 2 starters would be 23 or younger and a couple of them would be under 21.
There's a lot of room to get lost in the semantics of athleticism vs skill in this argument.
Basically, baseball being a ball and bat sport requires a different level of skill, than Football or Soccer. Not saying at all that they don't require SKILL, because they do, but it's like we need a different word to quantify the ability of using your own body for the sport vs using equipment and the degrees it increases things.
Hockey isn't a bad example, using equipment and you have to factor skating ability in as well, but Hockey also has a minor league system where most players go develop from like 17 until 21 ish. The ones that hit the ground running at 18 or 19 are the prodigies.
With baseball you've got the equipment factor, the athleticism factor, but then you also have a higher degree of mental factors too. Youve got angles striking the ball, mechanics of your swing to achieve the desired angle of contact, the recognition to tell different pitches apart, and the spatial recognition of the strike zone. That's all just hitting. Pitching you've got a whole game within a game to change speeds, views, location, and keep a guessing game with the hitters going. In the field you've got situations depending on the scenario and runners, almost every ball is hit different, the sun, hops off the dirt or turf. It's just a lot more variables and stuff that players need to learn. You've still got some that speed through, the prodigies, that can hit the ML at like 19/20, but the vast majority are closer to 23.
I think soccer and football rely the most on athleticism. Football has a decent amount of mental required in that there are scripted plays you have to memorize, and some positions you have to be able to read the field and certain players (QB/MLB/CB/S).
All of them have their own unique blends, but I think baseball outdistances them a bit since the reaction times shrink so much once you clear 18, and as the talent improves on both sides of the ball theres a certain mental game you've got to be able to keep up with for pitching/hitting, it's almost a game within the game. It just takes a little more development than the others do. You even see it a bit in the prodigies really, cause the most elite and talented, the ones that can handle physically playing at that level in their teens, still don't hit their peak or prime for a couple years, they are still learning on the job while they are there too.