That’s where the talent pool comes from. The key determinant for how competitive it is to get in the door.
While it no doubt requires less talent to make the NBA the taller you are, this just means that the most skilled players will cluster at the lower end of the height range. These guys aren’t filtered until the very end of the journey to the NBA. Meanwhile the majority of superior athletes are filtered out of tennis during childhood, leaving more privileged children to duke it out with significantly less competition. What this means is that there’s no hyper competitive clustering at the ‘poor end’ of the pro talent pool. Where lack of height is a disadvantage in basketball, poverty is an actual (and very early) barrier to career success in tennis.
I would be very keen to know how many pro tennis (or golf) players grew up in poverty.