I think I'm pretty close to your take. Women in America are on average only two years younger than the men they marry, and there are enough decent men out there that it's not especially noteworthy when one acts, well, decent. So the comparative that makes Keanu newsworthily noteworthy as a "nice guy" is the stereotype of celebrities in general being entitled jerks and of other "older" leading men in Hollywood more specifically being a--holes who only date 20-something supermodels.