It's one of those food mysteries to me. According to Tsuji, teriyaki sauce is no more complex than boiling equal parts soy, mirin, and sake, with a bit of sugar. It works best to finish cooking the meat or fish in the sauce and then deglaze, but still delicious to dribble that simple sauce over grilled or steamed fish, meat, tofu, or veggies.
I'm no cook -- but why does restaurant teriyaki almost always taste worse than that simple method? Why do restaurants make it gloppy with cornstarch or other thickeners? Is there some reason why the simple method doesn't appeal to American tastes? I have had amazing teriyaki (such as at the former Tanto in downtown San Francisco, or Minako in SF's Mission) but they are the exceptions.