Skyrim is an open-world RPG with a story, Fallout 4 is a story-driven RPG with an open world.
The first time I finished the main story in Skyrim, I really just wanted to clean it out of my quest log. I was massively over-leveled for it, having done lengthy quest chains for multiple factions, as well as an insane number of one-off quests and general explorations. When I finished saving the world, I still had at least a dozen quest chains queued up, plus another dozen miscellaneous quests. And I knew about a bunch more that I hadn’t even started.
The first time I finished the main story in Fallout 4, it was because I really didn’t have any other quests to do. There were the randomly-generated “cleanup on aisle one” quests from Preston, the occasional chatty NPC handing out tips, and a few location-dependent radio signals, but the vast majority of them were isolated quests in a single location. Some have multiple stages, like the town of Covenant or helping Travis, but they’re still pretty short. Even the main faction quest lines are pretty short and straightforward, and there are fewer factions to choose from, which largely become exclusive after you reach a certain point in the story (“if you accept this quest, you will become enemies with faction X”).
Recording voice for every quest option, for both PC and NPCs, is likely the main reason there are fewer quest chains, but the quests that do exist also tend to be less discoverable. For instance, I was inside a building when the (spoiler) crashed, and had no idea what Nick was chattering about. I wouldn’t have gone looking for it if I hadn’t seen it mentioned on the forums. Ditto the full suit of the best power armor, the gun that doesn’t need reloading, Vault 75, etc, etc. I still haven’t made it to the Atom Cats garage or the USS Constitution.
Bottom line, Fallout 4 works pretty hard to keep the main story moving, and if you stay on the rails, you may not even know how much content you’ve overlooked. Skyrim, even before the DLC, gives you so much to do that you may not even remember that there is a main quest.