If nobody in the game world wants anything, why even play? Player characters usually have some sort of motivation, whether that’s “save the blacksmith’s daughter” or “get treasure” or “seal away the ancient horrible evil before it can be unleashed upon the world”.
NPCs and monsters need to want things, too. What are the owlbear’s instincts? What does the evil abbot want? What are they doing before the adventurers come along? If the adventurers never show up, what will everyone else do?
Theme parks are a lot of fun for many folks in the real world, but they’re not really places for adventure. You show up for a day, experience the things that are there, then leave. At no point are you assembling any sort of narrative or trying to piece together any sort of emergent experience.
But RPGs aren’t like that. They have more relationship to a story than to a roller coaster. Players will poke at things and do the unexpected, which is very much not the intention at a theme park that presents a carefully crafted experience and (if only for safety reasons) does not allow you to go outside of it at all. There’s only one way to ride a roller coaster.
Note, this is different from “railroading”. You can have a railroad adventure with or without interesting NPCs, and you can have a sandbox adventure in the same way. We’re not talking about the players’ ability to decide what problems to deal with or how to deal with them. Instead, we’re talking about whether the NPCs are doing things other than just reacting to the PCs.
In RPGs, if your world has some dynamism, you have an idea of how the world will react when players inevitably do something you didn’t account for. If your NPCs have motivations and plans and desires, you can look to those to decide what happens next or how they behave in new circumstances.
Verisimilitude also comes into play here: in real life, nobody just sits around waiting for somebody else to do something. Every person has agency. NPCs who have no agency whatsoever will not feel “realistic” to the other folks in the game.
Let’s look at some examples: in Out of the Abyss, the starting scenario involves the PCs as captives in a drow outpost trying to escape. The leader of the outpost is a priestess who has spurned one lover after a horrific encounter with an ooze and taken another. Another junior priestess is there, and she’s related to the new lover. All four of these people want things (power, revenge, lust, whatever) and will take actions to get them. The PCs can gather information about the NPCs’ motivations and make plans accordingly. If they do nothing, then the DM can still make things happen. Whatever the players do, Velkynvelve will not be the same after a week or two. OotA has some problems later in the adventure, but the starting scenario is stellar.
On the other hand, we have Lost Laboratory of Kwalish (minor spoilers follow). The PCs go on an expedition into inhospitable mountains looking for a famed inventor. When they find him, he’s not doing anything. If they never find him, nothing will happen. Similarly, the other major location has some weird monks, but there’s no pressure whatsoever. The monks sit around until the players arrive and only react. The PCs could leave for a week or a year and come back and things will basically be the same.
Find the way that works for you, to be sure. Maybe you take some cues from Apocalypse World threats, or maybe you just keep a list of each person’s ideals, bonds, and flaws. GMs have lots of different tools available. But the end goal is to make the people in your world come alive.