Because the camera is focusing on the reflected subject, not on the mirror or water surface itself.

A reflection is a virtual image: the light reaching your lens appears to come from a point behind the reflective surface. So the effective focus distance is the path from the camera to the surface plus the apparent distance from the surface to the reflected subject.

Example: if the mirror is 1 meter from the camera and the reflected object is effectively 1 meter behind that mirror, the lens must focus at about 2 meters, not 1 meter. With distant reflected scenery, that virtual image can be very far away, so autofocus may choose infinity.

You can verify this by focusing separately on the surface and on the reflection: focus on the puddle or mirror surface and the reflected scene will blur; focus on the reflected subject and the surface texture may blur instead.

So the correct focus distance for a reflection is the apparent distance to the virtual image, not the physical distance to the reflective surface.