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Melodies are quite simple, conceptually -- there are only 12 notes.Whereas for chords, I couldn't personally begin to count them, but according to the first source which popped up in Google, for four-note chords, there are 495. And they're incredibly complex, with whole arguments (and probably dissertations) on how to properly categorize and name them and how they function.
That's irrelevant though.
The "complexity" of chords is just the result of a combinatorial explosion based on a few simple rules (adding this or that note of the scale, inverting in this or that way, etc). In fact any random cluster of notes one can hit (e.g. by throwing a bowling ball at their piano's keys) is a chord and can have some sort of name. That doesn't name it particularly useful, or anything worth much studying.
The complexity of melodies on the other hand, is much more esoteric and intuitive (you know it when you hear it), and thus much more difficult to write a great one.
>(Also, why compare "good melodies" with a "generic chord sequence"? Better to compare good with good.)
Because a good melody would be cherished by everybody, whereas few care for a good chord sequence (sans melody) except music nerds.
>Example: hum "The Girl from Ipanema" to yourself without harmonies -- it's the most boring, repetitive thing ever.
That's BS, I'd say. In fact, solo performances by monophonic instruments and a cappella singing wouldn't be interesting either if that was true in general (that no harmonies = boring).