The point of the size of the teams is a good one, but, as Moxie said, guys above #20 have great teams. Hell, you make around 50.000 US$ for winning just one round at a major. A guy living in the 10~20 range can afford a hell of a team. And when one guy starts calling attention, there´s no doubt opportunities will smile for him. I am pretty sure one guy like Kachanov, for example, who is still around #40, being a good prospect might already a pretty good team around him.
And those guys earned all that they have, probably beating people with more resources on their way up. I am sure all this structure wasn´t there on the 70´s, but surely was there late 90´s and in the 00´s.
Anyway, the question is legit, and if one thinks things are too unfair, only one way to solve: change the distribution in prize money. Interestingly enough, we have seen some top players pushing for it.
Edit: Challenger tour paying better would be of great help.