Because it's not a club championship, it's what the NRC should look like. And if the NRC did like that then this conversation wouldn't be occurring.
The clubs in parts of Sydney, parts of Brisbane/Qld, Canberra, Melbourne and Perth are all directly affiliated with a pro/semi-pro club. Players from the afiliated clubs are brought up into the pro/semi-pro club as needed and return to the community clubs when not required. Subbies and junior clubs within the geographic catchment should be all under the same umbrella, not off to the side in petty kingdoms. It's quite mad that in 2019 district, subbies and junior clubs in the same suburb are affiliated to different bodies.
When I went to the the WRU website a phrase which struck me was
Welsh rugby is aligned at all levels
http://www.wru.co.uk/downloads/WRU35WRUStrategyBookletv8LRE.pdf
Not a statement that could be applied to rugby in NSW or Australia where we have a tangled mess of overlapping and competing bureaucracies jealously guarding their patch
Note the importance placed on having clubs inside the tent as part of the solution, not outside the tent as an imagined problem.
I don't think a 'club championship' is the answer for many of the reasons outlined by dru.