Something else just hit me as to why a show may not use alternate announcers: in addition to everything being done in post, a lot of shows knock out their seasons over the summer or in a month or two, so it's not like Joey Fatone* has to worry about whether Chris Kirkpatrick can fill in for him this coming Tuesday, with Lance Bass announcing on Thursday.
I'd love to see more Johnny Olson or Charlie O'Donnell types, or hell, see Randy and Alan Kalter get more gigs, but I'm just happy that some game shows realize that announcers help set the mood just as well as a theme song. If the announcer has a set of pipes that brings an air of drama to the game and an extra dimension, then I'm fine. I'm not going to become a snob because they picked up a goofy shock jock sidekick or, in the current case of The Tonight Show, a voice talent who does work on Family Guy, among other shows.
*/Gotta admit, that was a left-field choice for Family Feud
//I really need to stop twiddling my thumbs and get into voiceovers