As the brake pads wear they transfer material onto the disk.
When they resurface the rotors they aren't doing it to make them smooth, they're doing it to remove the glazed material and provide a fresh face for the new pads.
Without that step the new pads ride on the glaze which doesn't have the same friction level as a clean disk, so the new pads feel "weak"
But as suggested, the machine to resurface costs thousands, and rotors are cheap so most shops have shifted to require pad/rotor replacements.
A less thorough alternative would be to use a Scotchbrite or Emory cloth to give the old rotors a some what better face, focus on the swept area where the pads make contact.