Tee times reserved for member + guest(s) play, and days or half days set aside for outside play (outings) are increasing at many even very private clubs.
The revenues from guest fees and outing charges can obviously be quite significant. At "name" clubs, the demand for access is fairly price inelastic, and charges will creep up steadily.
Food and beverage charges and pro shop merchandise sales associated with outside play are also welcome revenue "boosters".
The perennial battle to limit member dues increases, and even help fund capital projects, is the driving force. The background noise you hear is the sound of teeth-gnashing by those who want unrestricted access to their first tee seven days a week, all season long.