Because with the vaccine, people recover more quickly so they are in the phase where it can be transmitted for less time. The vaccine DOES NOT prevent transmission, but because people get less infected because their body fights it better, though it does not completely prevent tranmission, it reduces it greatly. And if all the fools get vaccinated, the virus will slowly be eradicated. If people are less symptomatic, of course the chance of spreading by coughing is reduced. You should already know all of this after close to 2 years.