What does it matter that there's (four-digit-number) of dungeons and towns in Daggerfall if I won't visit most of them and they're mostly identical anyways?
Replayability. I could start a new game in Daggerfall right now and have a different experience from my previous playthroughs, which would keep me interested enough to get enjoyment out of the game two decades after I first played it. Whereas I could start a new game in Morrowind right now and more-or-less know everything that's going on in it due to having played through it a couple of times before. Mods aside, Morrowind has nothing new to offer and so is boring to me, despite whatever nostalgia I may have for it.
So trying to get back on topic responding to the OP's question, here's how I look at it: Content and gameplay are two separate things. With gameplay, either you like it or you don't. Content, however, is a consumable resource. You may like the content, but once you've experienced/consumed it, it is basically predictable and experiencing it again would be repetitive, so you become bored with it for a while. If a game has limited content, even though it's all unique, eventually you'll exhaust all of it and no longer enjoy it, however much you may like the game itself. But a game that can take some simple content and present it in a way that appears to produce a lot of variety and remains not completely predictable (i.e. random procedural generation), this can allay the feeling of repetitiveness enough to allow you to continue to enjoy the gameplay even as you are technically consuming the same content.
Or to use an analogy, you might enjoy, say, pizza, but you'd be bored with only eating pizza with the same set of toppings every day, so instead you vary the toppings to keep eating pizza enjoyable for longer.
Skyrim and Fallout 4 both have more unique content than Daggerfall, but it's exhausted or fully experienced after only a few playthroughs (presumably the same can be said of Mass Effect 3, but I've never played it), whereas Daggerfall has so much content you're unlikely to ever exhaust it all, even though most of that content is just repetitions of a small number of themes.
It's for this reason why I can put over 200 hours into Crypt of the NecroDancer despite it only having less than two hours of story content and four dungeon themes, but got bored of Hammerwatch with its comparable amount of content after only about ten hours over two-and-a-half-ish playthroughs, even though I enjoyed the gameplay in both. Hammerwatch, with its fixed content, became predictable and stale, whereas NecroDancer, with its random procedurally generated content, remains exciting.
tl;dr Copious amounts of similar content makes for better replayability than limited amounts of unique content.
Or to state it as more of an academic hypothesis: gamers such as myself and the OP desire content experiences that are new while being paired with familiar gameplay we enjoy. Thus the length of time in which we can enjoy a game is directly related to how long the game can keep providing new experiences, regardless of how much it reuses its content.
Of course, some gamers prefer complete repetition, i.e. grind. My argument doesn't apply to that subset of the culture.
Note that Daggerfall being old never substantially enters into my argument.
On a related tangent, I wonder what replayability Stardew Valley would have if it used more random content (i.e. crop prices and attributes, NPC likes and dislikes, etc.). This also makes me wonder how much of an impact shared experiences have in modern gaming owing to social media, but that would be another topic.