Never played any Metroid, unless frequently using Samus in SSB counts. (It doesn't). Hmmm, that's the big one that came to mind. I've never played any Half-Life even though I really want to and own them. Just haven't gotten around to it. No Silent Hill or Resident Evil either, though that's by design since I just do not do well with survival horror games. I feel like I've hit damn near everything else at least once. I've played Zelda but I never got into them as a kid because I couldn't figure them out. I'm appreciating them more now but I just don't have the time to game like I used to and I'm more of PC man these days, so. I've played a shit ton of Mario in my day. Actually I've never played a Tomb Raider game. They just don't look particularly appealing to me. Oddly enough I've never played Prince of Persia despite owning all of them. Steam Summer Sales kill me. No Duke Nukem if you really count that as a big franchise. I was too young for it in its heyday and they tell me the new one is garbage. I guess this is a good enough list.
So, as for the whys. Metroid is a weird one that passed me by. I had a Gamecube for the entire life of the system and never played Prime. I might have to rectify this issue shortly. Half-Life is just a time issue, I'll get to it eventually. Like I said, Silent Hill, Resident Evil, I can't do it. Scary games aren't my thing. Tomb Raider also just kind of passed me by without a glance. Duke Nuke the same.
Metroid and Half-Life I intend to play. The rest, unlikely, unless they pop up absurdly low in a sale or something.