That's true, but....
Key word is TIME. 80-90% of the time they are feeding nearly 100% below the surface. The other 10-20% of the time they are feeding nearly 100% on the surface.
(Actually, it's probably more like 50% of the time a fish is feeding below the surface, 5% of the time it's feeding on the surface, and 45% of the time that fish isn't actively feeding, but, same point applies).
The point being, let the fish tell you what to do. If nothing's rising, go underneath. If fish are rising, you are more likely to catch them on top. Fly fishing is all about figuring out the situation and adjusting appropriately.
And while the percentages are all true, I catch far more than 10-20% of my fish on top. I think many do, when averaged over a full season. Because #1, the topwater feeding times are fairly predictable and many of us choose to be in those places at those times. And #2, when you hit that situation, you catch more fish. It usually means a hatch is on and fish feed more aggressively during hatches. You can see them, so you know EXACTLY where they are at, what lane they are working, and likely exactly what they are feeding on. You also see last minute rejections and can figure out why and adjust, like a game of "warmer and colder" leading you to what you need to do.
The best wet fly fisherman and nymphers, and even streamer guys, play the same game, by predicting where and when fish will be aggressively feeding, and on what, and trying to match that with fly selection and presentation. It's just harder to do, because you have to infer everything. You can't see them so it's not as easy to see that you need to drift it another 5 inches out, or start a lift another 2 feet downstream, etc.
For me, anyway, my luck on a stream, even when nymphing or wet fly fishing, improves greatly after hitting a good hatch there once. You find out where the fish are. That current break where they line up in this pool. When you come back another day and there's no hatch, well, the fish are generally feeding in the same lanes, just deeper, but now you KNOW those lanes rather than having to guess by reading water.