"It’s very common to find seared ahi (tuna), which I love. However, even though ahi, salmon, snapper and other fish make good sushi (raw), it is rare (ha ha) to find seared anything-else. Why?"
Dave raises a great point. Is there something unique about ahi, or is it just customary?
I think there are two reasons ahi is commonly seared. The first is that the tuna steaks used for seared ahi are very low in fat and can't tolerate even slight overcooking. Unlike salmon, which remains moist whether cooked medium or even well-done (hence its frequent appearance at large catered dinners), tuna becomes chokingly dry when cooked anything past rare. Searing is one of very few ways they can be cooked.
The second reason is the fresh taste of cool, raw tuna. It tastes like the sea. Pair that with a seared outside, crusted with sesame seeds and some soy, and you have a unique contrast.
Seared ahi requires extreme heat, to brown the exterior without cooking past a couple millimeters of depth. That's tricky to do in home kitchens. Alton Brown demonstrates how to turn a charcoal-starter chimney into a blast furnace. I tried it and it works, though I usually use cast iron on the stove top. I have also tried a blowtorch.
I very often cook salmon so it's rare in the middle. It's the same idea as seared ahi but not quite the same doneness profile since the goal with ahi is to have it mostly raw with just the outermost edge seared; with salmon I just want it rare or medium rare, not necessarily raw.
By the way, the way to sear salmon is with skin-on. Crispy skin on moist salmon is da bomb.