Its the Morrowind vs SKyrim example.
Morrowind has replay value, because replays are varied and unique inherently in its design. Skyrim can have replay value, but everything about it is designed to be a ludicrously long grinding singular do-everything playthrough, not a unique experience per replay.
In Morrowind's case, their are logical barriers (like needing to be a mage to rise in the mage guild) and questline designs that promote conflicting interests. Most of the content (and the endgame) will eventually be overcome by a natural (somewhat, levelling is well-documented in its jankiness) progression, that doesn't require doing all the content over and over repeatedly to achieve.
Skyrim specifically eschews any barriers, and while dialogue plays lip service to questline conflicts, no one will ever care that you're an archmage in the magic-despising Companions, or a celebrated Imperial General killing the Emperor. Challenges in the world keep scaling until you unlock the upper echelons of perks, a task that involves constant repetition over hours. The perk system is even designed this way, with the final perk requiring the absolute full grind (contrasted to say, Super MArio Odyssey, where the equally inane number of moons existed, but you didn't need anything close to all of them). Then you have the trophy design, where the designers have further mandated that everything be done with one character, in direct opposition to the concept of replays.
There's various factors of course. Trophys are an obvious culprit, though to my knowledge, devs don't have anyone holding a gun to their head making them choose bland marathon grinds as their trophies, or even assigning the platinum to "100% completion". "Biggers number = Better", so having thousands of collectibles and 7 billion planets with 1 million mile square maps is seen as the target, even if the collectibles are meaningless, and the terrain is procedurally generated nothingscape. Attempts to mass market also result in trying to merge accessible (or "easy" or "casual" if thats your preferred terms) gameplay with the anti-thetical concepts of deep challenging gameplay and expansive worlds, by just repeating the accessible content to make it seem grander.