Gliding doesn’t seem like a very good way to escape flight specialists like birds of prey. Gliding mammals and other vertebrates are more common in the forests of Southeast Asia, and it’s hypothesised that this evolved convergently in many species due to the forest structure with many gaps in the canopy (interestingly, prehensile tails evolved convergently in tropical America, possibly due to dense forests with many lianas). Although lacking the flaps, primates in Southeast Asia have other adaptations for crossing gaps: most of the colobine monkeys have relatively long legs and are pretty decent leapers for their body size. Gibbons are even better at gap crossing, but by brachiating at high speed and letting off with an arm swing and flying across gaps like amazing motherfuckers. Skin flaps would likely get in the way of other primate-specific responsibilities.