At many popular hiking areas, hikers are advised to stay on trails so as to minimize impact on the environment. In Hawaii, most often, the impact is reversed and the smart money is on the flora and land to deliver a blow to any hikers foolhardy enough to take off cross county.
Hawaiian mauka (mountain) routes almost always follow a ridge, or head up a narrow stream valley, rather than following contours on traverses and switchbacks as they do in most mountain regions. Razorback ridges narrow down to a foot or so in places, with 1,000 foot drops to either side. The thick flora that buffets the routes disguises the hazard: you run the risk of stepping into a little turnout that turns out to be a tangles of greenery.
Hawaiians have been walking the islands for many centuries. If you don't see a trail, there isn't one. Follow the trail, and if you lose it, turn back.