Some do use chains but they are a packaging nightmare. They require oil pressure driven tensioners, delrin pads to take out slack, and overall can be worse than belts. The problem with a chain that is long enough to do an OHC, the tolerances are multiplied 10-fold over a cam-in-block design. As the chain wears and gets slack, the tensioners correct for it, but the delrin tensioners wear away under the friction from the chain, and the rate of wear on the links can lead to shorter life spans than the belts.
The big reason is money. Cadillac has a pretty good chain setup for its OHC engines. Mind you, its a very large, triple roller chain that is almost 5 feet long and has a lifespan approaching 100,000 miles, but its still not perfect. It requires a computer-sensored, servo driven tensioner that is complicated and expensive.
Belts on the other hand are easy. No special tensioners, so-so service life, they dampen valvetrain harmonics, and are light and easy to package.
You're right. Belts are a pain with their service intervals, but there are considerable trade offs that I don't mind. I just solve the problem by sticking with classic American V8s
Ironically enough, racers using American V8s often prefer a belt conversion on their classic iron. Of course, the exposed belt on a V8 is much easier to change than on an OHC, but the belt itself has some marked benefits. Suffice it to say that the belt is better in most ways from an engineering standpoint, except service intervals.