Two reasons I think
1. Nintendo usually utilizes their characters for spin offs moreso than other company. Mario Tennis and Mario and Sonic at the Olympics may be completely different than Mario Galaxy, but some people still just see Mario.
2. Nintendo's franchises have been around so long that it feels like the franchises are a lot larger than they are. There have only been 8 console Zelda titles (and four swords), but the franchise still feels larger than that because it's been around and it's been relevant for 25+ years. It's more a matter of constant exposure than milking.