I would say that height is an advantage up to a certain point in basketball, but tall people are not especially rare. Within the market of basketball players, you can find tall people who also have other skills, sometimes you find short people (Steph Curry) who have exceptional skills.
A better example would be Muggsy Bogues who was a full 12" shorter than Steph Curry and he could dunk.
Read between the lines. If all the players are tall, and all the coaches are tall, and the game has been played for more than a half century with that assumption... who knows how to train/coach a short player?
What proportion of people out there can learn to jump so high, even with extensive training/practice?
That said, a quick search of "training to dunk 5'6"" on Youtube brings up a number of videos.