A lot of it was intentional by those creating sports and leagues wholesale, some accidentally because of the realities of who had time for leisure, and even just some chance.

To do that though we can look at the big 3 sports in the US. Gridiron Football, Basketball, and Baseball, the first 2 have a huge presence at the college level with obviously a huge business having built up around it, while baseball except for a few places(looking at you ACC and SEC) really doesn't.

For some perspective its important to first understand how the sports came to be, and while there are of course disputed points about all the sports and their origins the consensus and generally accepted narratives are sufficient for our purposes.

Football.

Football is as you may be aware a spinoff of the larger family of ball games that produced Gridiron, Gaelic, Aussie rule sets, along with Rugby and Soccer, all emerging from that nebulous past of local variations of games first began to be codified in the 1800's at English Boy's schools. And being ever conscious of showing they were just as good most US colleges and universities had their own house rules that were often little more than excuses to haze freshmen for a few hours. But that is only fun for so long and understandably new opponents if a ruleset can be agreed to is a new challenge. So in 1869 a group of students at Rutgers in New Jersey invited a group from nearby Princeton for a game, the Tiger squad arrived by train, had lunch, shot some pool, and lost a close game 4-6 to the boys from Old Rutgers. The Princeton team beat a quick retreat home following the game after things got a little chippy and a small crowd of a few dozen watched the game that was far closer to modern Soccer than anything else, and promises were made for a second game in Princeton that was won by the home team 8-0 a week later.

The sport followed this pattern of one off games under home rules or agreed rules like that for several years. And in 1874 Harvard invited McGill from Canada down for a few games, including using the new Rugby rule set that McGill was playing with. Harvard's team liked the rules so much they began advocating for more usage of them, and convincing schools they played often and already had other rivalries(like rowing) with including Yale, Columbia, Princeton, and Rutgers, this is the origin of what is today the Ivy League! From there a man from Yale who would be first a player, then coach, then sportswriter, all while also basically controlling the new joint rules committee for the sport would enter. Walter Camp would essentially take the Rugby rules and develop most of the iconic parts of American football like the scrimmage and down rules.

With this new sport taking off at prestigious Mid Atlantic and North East schools many others began to want to get on board, Mid West schools like Chicago, Michigan, Minnesota, and Wisconsin joined with others to create what would become the Big 10 and began trying to prove themselves against the Eastern Powers creating large games that drew thousands in the 1880's and 90's while schools in the South and West tried to quickly catch up, often hiring coaches who had worked or played for the major powers. And from there with more schools playing and more media attention every year, College Football was established at least for a time as the premier level of the sport. This was also aided by many new schools and institutions being founded in the decades following the civil war, including many being supported by money from the Morrill Land Grant Acts creating far more spaces at schools of higher learning compared to the pre war years.

Basketball.

Shooty Hoops has a similar history as well. James Naismith was in 1891 had been the Athletics Director at McGill( look at them go!) and took over the same job at the then YMCA run Springfield College in Massachusetts. Asked to come up with a way to divert all the students' energy during the winter months indoors he took what he saw as the positive aspects of several games like Soccer, Rugby, and even Water Polo, and came up with Basketball!

The YMCA networks took a liking to the game and helped spread its popularity, and in the days before the NCAA these club teams were common opponents for nearby schools in many sports. And in 1898 Naismith took a job at the University of Kansas and began to lay the foundation for that blue blood program. The coaching population was also pretty heavily interconnected in the late 1800's, most being able to trace their lines back to that founding generation in the Ivies in the mid 1870's or elsewhere. For instance on staff in 18981 at Springfield was one Amos Alonzo Stagg, he also helped by suggesting cutting team size from 9 on 9 to 5 on 5. He had played for Yale football a few years before and was selected by Walter Camp to be among the first ever All-American Team. Stagg would be hired by the President of the new foundling University of Chicago to intentionally use sports excellence to promote the school. He would coach both sports(though with a HUGE focus on Football) and even become Athletic Director over a 40 year career, and was the first of what we would recognize as a professional coach in the modern sense.

So we see again that the game was created for and spread initially through the school networks, though the YMCA connection helped and also gave it an early out towards international play that we see has aid huge dividends.

Baseball.

Baseball then is the only of the 3 not created on campus. It too has its own murky origins in local rulesets and more ancient games. But for the US it began to see more exposure with local community organizations in the years before the Civil War. Notably with groups like the New York Knickerbockers in the 1840's and 50's. With groups of local community clubs banding together even then and the early professional leagues starting up in the 1870's.

In the end then the large roll that collegiate sports plays in mostly because for 2 of the big 3 its where they began. And with early hype, and interest drawing crowds already there was relatively little room for community or civic leagues to form that could compete in a meaningful way for many years. Hell even for professional leagues it took more than a few false starts to find a way to make things work and emerge from the shadows of collegiate sports which even by the 1880's and 90's had established themselves as a big business venture with paid staffs, huge stadiums, lots of media coverage, and more than a few corruption scandals. That head start would prove difficult to overcome, and then in many ways beneficial to the professional leagues as it erased the need to them try to develop a functional farm and development system.