Because they are technically window exit rows. But, as you correctly pointed out, Seatguru has them listed as red (undesirable) because they have LESS legroom and LESS recline than a typical exit row, and also feel rather cramped, with only a tiny LCD monitor in front to watch IFE.

What AA should do is make row 20 "Preferred" (meaning, for sale to anyone), and keep row 21 "Preferred Plus." Trust me, elites who know what they are talking about have very little interest in row 20.