>>12093891
Because we've experienced so many games nowadays that their level of gameplay is something you would find as a minigame meant to pass some time in other games or so simplistic that it's hard to view them as anything other than an extremely boring thing. Even basic flash games released 20 years ago that are meant to be like 30 minutes long like those zombie tower defense games are more involved and are more engaging than the overwhelming majority of those games.

A simple example is something like ET on the 2600. That entire game is falling into holes until you find enough objects to leave while you dodge the cops. There's nothing special to it, there's nothing interesting about it, it's basically wandering around aimlessly falling into holes hoping said hole has a flashing object in it and stretching your neck to get out while at the mercy of the cops randomly spawning in as you transition rooms.
How about Shark! Shark! on the Intellivision which is you playing a small fish eating bigger fish as you try to dodge a very slow moving shark?
Or Snoopy and the Red Baron on the 2600 which is just you flying around shooting stuff in the sky until you lose.

Games that are made explicitly to evoke an arcade-esque experience like Dig-Dug or Asteroids are timeless because from the get-go you understand they're meant to be survived in, gain as many points as possible, you very likely transition stages, and there is constant progress in how difficult it gets with each stage and how the game itself looks. Pong and Breakout are super simplistic, but they exist for score attacks so they're easy to pick up.

It's also because a lot of these games have updated versions that make the old versions entirely obsolete. Space Invaders, Breakout, and Tetris are all examples where the old version is so outdated it's not necessary anymore outside of nostalgia. You have Space Invaders Forever on modern systems, Breakout Beyond on modern systems, and Tetris Effect on modern systems.