Sandbox games have proven that they stand the test of time. Why do you think the sandbox model appeals to players even after all these years?
What's not to like about a model that lets you shape the game to your own image and at your own pace? Games like this are great, in my opinion, because they're all about the journey rather than the destination. I'm notoriously slow at leveling in MMORPGs, like Fallen Earth, because of this particular quirk. It's also why I really love games that come with design toolkits, they make it possible to dabble in my own creations.
Some of the games that have been most appealing to me over the years provided the opportunity to just go out in the realm and, within certain parameters--the boundaries of the sandbox--do things to experiment with actions and consequences.
SimCity, Civilization, and Railroad Tycoon immediately come to mind as computer games that I absolutely loved for the simple fact that I could build my own city/empire/industrial transport company and take them in whatever direction I wanted. Through the process of trial and error, I learned which aspects worked better together than others. You want to put industrial and residential zones next to each other? Go for it, if you want nothing but slums! Sure, you can deal with a rival civilization's leader using diplomatic methods, but maybe it'd be more fun to ramp up military production and crush him beneath your boot heels instead. You can take your railroad on a straight path from one city to the next, tunneling through a huge mountain range at great expense in the short term, but maybe that saves you time and money over the long haul.
You might succeed. You might fail, collapsing into utter ruin. That sense of experimentation and risk without any real, lasting consequences played to my love of asking "What if?" and running with it. The worst that can happen, after all, is that you have to go back to a previous save point before you decided to embark on that dubious war against the enemy.
It may also be useful to consider that sandbox games are often more conducive to a broader audience because they're like books that you can pick up or put down as time allows. Or, if books aren't your thing, they're like DVDs or a show recorded on the DVR. You can freeze progress while you take a break to have dinner with the family or you can treat it like a fish bowl, let it run all night, and come back and see how things are going in the morning. The game isn't governed by clan raid schedules--no one's yelling at you over the headset to get to the instance while you're fussing with the baby. You're not necessarily coerced into playing through a linear storyline for hours on end to get to the next phase while you're thinking in the back of your mind: "I love the immersion and I want to slay that dragon, but I DO have to work in the morning."
In the end, sandbox games become far more about the player, what they want to do, and how they want to affect the game worlds than they are about how great the game's design or dialogue might be--although it's a huge bonus when the game fosters that sense of sandboxiness while also providing a well-designed immersive experience.
They appeal to so many people after so long, I think, because no matter how old we get and how many games we play, there's still that mischievous/ambitious/inquisitive part of us that wants to build a castle out of sand and then pretend we're Godzilla, stomping all over the beach, yelling "RAWR!"