The appeal is that these games were made well before Elden Ring and Witcher 3; before 'open world' was mainstream. Every single NPCs had daily routines, quests were dynamically generated, and what you do in the game had consequences beyond pissing one character off. This made triggering specific quests or events difficult if you were just going 'off the hip', and made replayability a big feature. Because of these systems, there were several interesting "game breaking" issues, but these things were charming in their own right due to how new all these systems were put together with almost nothing like it.

In retrospect, not all the "game breaking" issues were truly understood at the time, and most are a consequence of several factors - the most common being that some quests activate behind the scenes and prevent other quests from starting, even if you haven't picked them up and added them to your journal. So it is possible to do mostly everything in the game with careful planning. But at the time, it really did seem like each playthrough was unique.

It is/was also highly moddable for its time. While it took a long time to detail every aspect of the game, today there is nothing mods can't do. Even Witcher 3 mods can't do a bunch of things that Skyrim mods can. And it's a good gateway into learning how to mod, and modding can be just as fun as playing. Some mod guides are so long it takes days or weeks to implement. It can get quite insane, with some people maintaining multiple 'mod versions': one to play (most playthroughs won't let you add/remove mods mid-play), one to test new stuff, and one to keep up-to-date with whatever mod guide/group they are following (you know, for fun....and the next playthrough).

But mostly it's nostalgia, like how some people like older Zelda or Final Fantasy games. Or how you might play that pointless cozy game you played a million times because it connects you to something deeper to what was going on at that time. We know TES games are pretty bad in a lot of regards, but graphics, gameplay, or story isn't what we are after. Hell, there are now adults booting up Minecraft because it's just the game they grew up with.