My guess would be because WGT already runs several (maybe 10?) weekly tournaments for credits, and many more throughout the month. These credits can be easily converted to passes, apparel, clubs, balls, etc... So why introduce another form of prize payment?
As for country clubs you can easily set up a tournament for your members with no prize pot, but let everyone know that you (or whoever the sponsor is) will gift the winner the desired amount of passes upon completion - we do it all the time with sleeves of balls. If someone wants/needs passes for Clash/Turf War or something like that one of the Directors will gladly gift some along as long as the member is contributing to the club.