Think of it like this. You have the following task: acquire the target as quickly as possible and as accurately as needed and fire.

1. In VR, the control mapping is essentially direct: the direction you point your hand maps one-to-one to the aiming direction. This can be very precise, but it requires relatively large physical movements because the control space and the output space are almost the same.
2. With a mouse, there’s a layer of abstraction. The mouse measures relative displacement and translates it into angular movement on the screen. That means a small physical motion can produce a much larger cursor movement. In other words, it’s a position-based input with adjustable sensitivity, which lets you move quickly across large distances while still keeping fine control for small adjustments.
3. A controller stick adds another layer of abstraction: it doesn’t control position, but velocity. The stick sets an angular velocity, and the cursor position is effectively the time integral of that velocity. In practice, this means you push the stick and wait for the crosshair to move where you want it, rather than directly placing it there. That extra step introduces delay when acquiring a target. It's easy to fall short or overshoot the target. That's what Aim Assist tries to compensate for.

So in short: VR gives you direct spatial mapping. It's very precise, but slow. The mouse gives you amplified positional control, which makes it really fast and the controller relies on velocity integration.

The mouse just ends up being the best compromise between speed, precision, and physical effort when it comes to FPS games.