And that's the way it mostly was until about 15 years ago when Detroit was made to get serious about energy conservation, fuel economy and emissions control. If the engine must be smaller and lighter, then the cylinders themselves must be smaller. If the cylinder must be smaller, then the valves must be smaller. If you could do that you could get the same horsepower out of a much smaller chunk of metal. Greater throughput of air.
Smaller valves mean less weight.
Less weight means less energy needed to open and close them. It also means lighter valve springs to hold them closed. All of which means that the engine can safely spin at a higher RPM which again, increases the power output for a given engine displacement. Many are still 2-valve for cost reasons. And now we are seeing some 5-valve production engines.