Tomic and Robson are not world class players
They have their faults, but you cannot deny they are world class players.
Also the game is developing. Tour level is far harder than challenger lever ever was. This is why it was easier a decade ago for youngsters to break into top 100. Now players ranked 50-100 are so much stronger that they can hold off and defeat youngsters that are trying to come up.
So getting WCs to give better experience to youngsters for the top level has big advantages over players who always play qualifying (which is filled with challenger level players) and never experience to what is required.
There are many recent examples of very good junior players who tried to transition by only playing challenger level events and get there with their ranking, but now they just got stuck there, they just cannot make the step up anymore.
If you look at Ash Barty career:
Very good junior at Aussie tournaments
Lost in Junior Australian Open (i think WC)
Dominated juniors internationally
Ok-ish ITF results so far
Lost in Australian Open
Dominated Aussie 25ks / Still poor in doubles
Lost in French Open
Dominated Nottingham $50k / Just won first double title
Lost in Wimbledon
Cont. Dom. Aussie 25ks / Dominated doubles
She has developed so much because of her WC chances. Now she knows how to play at Aussie Open, so she has a good chance to win a round this time.