I think if there was one game you could single out as the best computer game ever, it would be Minecraft. Out of the box, there are several ways to play it: creative mode where you cannot die and have all the resources you need to build whatever you can imagine, and survival where you need to mine and gather your resources before you can build anything and at night monsters come out to kill you. Then in survival you can just keep collecting resources and build whatever you can imagine, or you can try to finish the game by killing the Ender Dragon, which involves travelling to two different worlds and surviving some pretty tough challenges. Speedrunners can do it in 20 minutes, I cannot do it at all.

Then there is a whole community of online servers which can run a variety of plugins that completely change the game, from a wide variety of minigames that may not involve any mining or crafting at all, to a game where you're resource-starved on a floating island, to a game where the resources available are totally different.

I rarely play it, but I can totally see why my son, nephew and all their friends are addicted.

There's RPG-ish & enchantment mechanics to build up better tools & armor, different biomes to get different resources from, NPC villages to trade/upgrade/protect, and various enemies with varying drops, including in bases/dungeons/strongholds.

The common game loops are "I want to build X, so I need to find area Y to defeat mob Z to get the resources", and "I need to mine resource X, so I need to get resource Y to build tool Z to be able to do so", as well as building resource farms and some rudimentary automation.

So there is enough structured activity to drive actual gameplay beyond just having a pile of virtual blocks to stack, but it contains an additional unrestricted sandbox mode as well.

For some people, the sandbox mode is where the fun is. They ditch entirely the survival part of the game and simply like to build things in creative mode. Some people build houses and voxel art, others create complex mechanisms up to functionnal logic gates and computers, and others just like to explore the fractal world.

You won't really get the appeal until you try it yourself. It's $20, easy target for a lazy Saturday afternoon.