In some games they can be the same, and in some they are. But in many games, that doesn't make sense. Maybe the NPC has a role, e.g. a police officer needs to wear a police uniform. Or maybe the NPC has a personality that the clothes help express. An innocent religious girl will not wear revealing outfit and a rebellious daring one probably will. Taking that away with a slider could really deform what the game is trying to express. Especially if NPCs don't get enough screen time, their looks are important to convey their character.

PS: There is also a technical limitation. It's not just artist effort that focuses on PCs. They clothing of playable character will often be more complex. Perhaps it is more detailed. Perhaps it can be combined (different top, bottom, accesories). Perhaps it deforms around character creation sliders. Perhaps it has physics enabled. All of this is extra load on the GPU and in many games is simplified for NPCs. So giving NPCs PC outfits may kill performance. If I again bring up GTA SA as an example (because I know a lot about how the game works under the hood), the main character CJ is the only one that has clothing handled separately from the player model. For most NPCs, their clothes are part of the NPC asset and can't be swapped as that would take extra effort and resources.

Now non of this means it is impossible, but for most games, it is just not worth it.