I can suggest an explanation - conservation of mass/ energy.

let’s assume that Lt Riker’s ship had to use up at least 40kg of antimatter (and 40kg of matter, but that’s cheap) to generate the energy for that transporter operation that produced a whole extra 80kg Riker.

that’s a bare minimum with perfectly efficient engines and transporters, so it was probably well over 70kg of antimatter.

the Dominion decides that a few years of growing and training new troops is far less expensive than spending so much antimatter.