Fame
We're willing to bet some of you knew the sportscasters we've mentioned so far and some of you didn't. We're also willing to bet those who did recognize them are, in general, the bigger sports fans. That ought to pretty much answer the question of fame. Costas is probably the most famous sports announcer because he is known to appear on TV in non-sports related situations.
This means that at least occasionally he is seen by groups of people who have not had seven beers in the last two hours. This is also true of Joe Buck, to some extent. Costas has also been the NBC Olympic broadcast host for over twenty years (yes, we realize the Olympics is about sports but he's the host, not the play-by-play guy).
He's had bit parts in movies, done commercials, product endorsements, pretty much everything but run for office. But he's the exception.
Sports fans know the broadcasters they hear regularly, both local and national, but that's normally the extent of it.