QNH: I think it is a misconception that RA flyers do not get their PPL because they think it is hard. It has to do with cost.
It costs more to gain and maintain a PPL, which seems pointless if all you do is fly RAA aircraft! Paying annual fees to two arganisations, for instance. BFR with a PE in a hired VH plane is more expensive than with an RAA CFI in your own plane, etc. (remember: 1 in 3 RAA flyers own their aircraft!)
As PPLs, you can ask why not just get your PPL? Other than the answer given above, you can look at the question from an RAA pilot: why can non-PPL gliders go into CTA but somehow we can't be trusted?
As for me: I do fly into CTA using the special exemption we have here at Parafield. The main reason I would like to see the CTA endorsement is that we might see a more active RAA scene here. After all, one of the best things about flying is hanging out with similar minded friendly folks!