Once you've seen everything the game has to offer, it starts getting boring quickly. Like watching the same movie over and over. Some you can watch over and over because they're that good, some you watch once or twice and you're done.
Now to your question of it being too hard to beat good players in an older game, that does have an effect. You might find it difficult to get into Counter Strike or Team Fortress 2 these days because the people still playing are really, really good. They've been playing the same game for a decade.
This is then considered the "meta", most efficient tactic available, and in some communities, it will be heavily frowned upon to deviate from the established meta as that's seen as "throwing the match".
That's why a lot of multiplayer games are really fun right after release: Most players are new with no clue what they are doing, everybody is enjoying themselves even if they are messing up, they are still having fun exploring how everything works.
But as communities mature, and more people understand the mechanics at play and how best to exploit them to their gain, the community becomes more "mature" and competitive. Which is completely at odds with "messing around for fun", a process most new players learn by.
Not disagreeing with you, just adding on.
Playing an unpopular indie game (or even a game that's >8 years old) is kinda lonely. Yeah its fun but who am I going to talk to it about? Playing Fortnite or Overwatch or even a new AAA single-player game feels like being in the cultural zeitgeist.
There's also a glut of indie games like Into the Breach,Celeste, Dead Cells - if that's more you're thing!
There are oodles of great single player games being released still, and the enteryainemtn value of them is massive - games like Red Dead Redemption have o er 40 hours of content just in the main storyline alone!
If all there is to do is to get achievements and better loot, then players will tire and leave.
If there's community, ways to express identity, and so forth, then players will keep each other engaged.
Fortnite doubles down on the latter by having an in-game store with time-windowed products and other mechanics that heavily leverage FOMO and social anxiety.
What games are you thinking about where this applies?
Many of the most popular games with which I'm familiar (Overwatch, PUBG, Starcraft II, DotA, League of Legends, Counter-Strike: Global Offensive, etc.) seem to have very high skill ceilings. That is, high-level competitive players in those games are way better than the average gamer.
I think a big way these games bring in players/stay current is by having a very active esports scene -- which only really works when you have "pro" quality players.
With the exception of time-gated achievement games (most MMOs, for example), the trend I see is a low barrier-to-entry but a very high skill ceiling.
One tactic that seems pretty common is to get people in-game quickly (f2p), teach them the core gameplay mechanics, then make it a very time-consuming grind to achieve the highest levels of play.
Consider that in Fortnite a junior player can outlive and outlast a seasoned player by avoiding conflict, and score reasonably well.
In Pokemon Go and other similar mass market games, skill is an after thought at best; it's all about the balance between engagement with your friends and community and grinding levels and awards; intrinsic versus extrinsic.
The prolonged death of arena shooters is the evidence of the importance of skill.
Most games will put players of similar skill level against each other, so the fact that there's a cohort of players who are much stronger than you can be totally invisible to your experience of the game as a weaker player, even when that delta is getting larger over time. Whatever skill level you happen to be at, there's probably someone else there too.