I believe it's because Chuck Blazer saw that the US had few if any opportunities to play decent-quality international games and created the Gold Cup to be an answer to that, since we're essentially permanent hosts.
Even if we bring back guest countries as competitors, it's never going to "mean more". There are no WCQ bids on the line. There are 3 meaningful national teams in the confederation. I'd rather have it continue as it is, because it's entertaining, you see our top prospects and a good number of A-team players, and what the hell else would the team do during the other years' summers?