I think that happens because some developers doesn't realize what where the core features of a game that gamers liked,and make new games in a series that miss those fundamental features that shaped the experience the players loved and expected from the sequel.
This most often happens on FPS series,but it happens sometimes too in games of other genres.
Look for example on the 2009 Wolfenstein game and Duke Nukem Forever:
* They scrapped the health meter / medkits system.
* They got iron sights
* They turned to cover-based shooters.
Practically they copy-catted Halo and Call of Duty,but fans of these series weren't expecting to play Halo clones,they wanted to play Wolfenstein and Duke Nukem.
Homogenization is unfortunately an evil truth,and when developers scrap original game ideas and say "lets ignore the fanbase we have and copy Call of Duty so we can steal some Call of Duty fans" it ends up bad,because Call of Duty fans ALREADY HAVE CALL OF DUTY to play and won't care for a Call of Duty clone,while the original fans won't get what they wanted and expected either.
That's why these games flop and fail,but lots of developers keep discarding unique and tested working gameplay formulas and instead make clones of other games thinking they will steal the players of another game.
It is even more obvious on the MMO genre,where everyone copies WoW,and everyone fails one after another.
Some are just so stupid they can't understand that the fans of a game are already fans of that game and won't leave it for a game that clones it.
Beyond the console vs PC stuff, another thing for me is that I was/am used to having more games that were not one-shotters. That is, games that were still fun upon lots of replays, even if I played them almost exactly the same way each time (same party group, same general alignment/faction/strategy, etc). There was something about them that led to me wanting to do so, such as "can I replay this area/level for more efficiency now that I know what I'm doing" (or whatever) that made it interesting/challenging even tho I already knew the "answers", level layouts, what to expect in the boss fights, etc.
But I get that feeling less and less often now. I may enjoy a game, but when I'm done, there's little desire to replay it. Even RPG's where I can play with a different chr. class...the game itself isn't interesting enough for me to actually chug through it repeatedly to check out how it plays if I change alliances or class or use a bow instead of a sword. Why this is, exactly, I'm not sure. Less feeling of freedom to experiment (eg more linear in scope/options, even if you have a giant world to explore or 3 dozen campaign maps)? Dunno. Maybe others have explanations for what might be missing. There is, of course, nothing wrong with one-shot games, but they don't tend to be the ones I call "classics" or absolute faves because they aren't as long-term memorable. Gets people to buy more games per year I guess. Maybe I just expect too much value for the dollar.

You scratched the surface of another wound of modern games.
I see what you say there. Most modern games lack replayability,and do you know why this is ? Because of over-simplification.
There is a trend for simplifying games,but sometimes developers overdo it.
If you ever played Quake 3 or Quake Live,you should know that there are many techniques in that game like strafe-jump-fast movement,rocketjumps etc. To master these techniques it will take you lots of exercise,and once you know how to do them,the possibilities of what you can do in the game and where you can go expand,and that gives you a reason to play through it again and try out a different approach.
My point in the Quake example,is that Quake was a game that required practice to become good at it,a game that rewarded the player for spending more time in to it.
Nowdays the simplification trend removes such techniques from gameplay of modern games because they are deemed "too complex" for the average player to understand how they work. It's basically like saying "hey I consider most of gamers to be stupid,so I'm going to tailor my game for stupid people,so sorry for taking out the fun for the rest".
It is like if a chess game was released,where all pawns could only move with the same pattern and the same number of squares.
Sure it would make more people being able to get to know how to play it quicker,but it would ruin the game and it wouldn't be chess anymore,and all the fun for real chess players would be gone.