Well one thing is the way the game is played. With football, it is such a physical sport that you dont have time really develop guys because of the beatings those guys take week in and week out. Running backs typically fall apart when they hit the 30 year old mark. The only positions where you can be successful for a long time are qbs and kickers because they do not typically take a beating. College is basically the minors for football. You dont have the body or time to go from college to the minors to develop because one hit in football and your career is done. With baseball you can see guys playing well into their 40’s so they have time to develop their game before going pro.
In basketball, athleticism goes a very long way. There is only so much you can learn after college ball in the pros that you haven’t already learned. Big man can learn post moves yeah but they typically can contribute right away with rebounds, put backs and easy dunks. Guards are pretty much who they are going to be when they come out of college. They are good shooters who only get better with time, passers and those you get the whole team involved really happens with playing with the actual team you will be on and not a D-league team. Most basketball players fit a typical mold of a player. And also with their bench being as small as it is, usually the people in front of the incoming player is not going to be as good as the college player.
Now with baseball, it is more of a finesse game. Sure athleticism and raw talent do go a long way but fine tuning of your swing, or your pitching mechanics is what makes you great but it also takes time and patience. Hell the best player ever Babe Ruth wasn’t a shining example of athleticism. It takes time to turn the raw talent into good baseball mechanics and it easier to take those guys in early and teach them how the coaches want them to be taughtand not some highschool or summer league coach who probably is giving them bad advice.
It’s really hard to change a good swing that gave you good success in high school or even college into a great swing that can compete in the pros, especially when they’ve probably had that same swing their entire playing career and muscle memory is a hard thing to change. Also the season is REALLY LONG. Looking at pitchers alone, a 5 man rotation will probably have a lot of changes through out the year from injuries, guys not doing so well, weird scheduling that makes them call up guys for a game or two. That’s starting pitching alone.
Now take into account most teams will have a bullpen of 7 or 8 guys that only go out between 1 to 5 times a week and you have a lot of potential for injuries. Also, guys just lose their spots to better players in the minors. Now take into account a 25 man roster over the course of 162 games and you can see why teams need minor league teams. Also, like i said before the lingevity of a baseball career is what keeps younger players in the system longer. You may have a good 20 year old prospect but the position he plays is taken by your all star caliber player. Its not like in basketball or football where you can sub him in and sub back the other starter. Once a starter in baseball is subbed out he’s done for that game.