Because the chance of infection is greatly reduced when two vaccinated people come into contact with one another.
Let me explain some more.
What we do know is that covid recovery time amongst the vaccinated testing positive is more than halved due to milder symptoms. Knowing that this leads to a much shorter period of the vaccinated person being infectious, when a covid-free vaccinated person comes into contact with them the odds of them getting infected are greatly reduced not only by this but there now being two barriers for the virus.
You can still get tested at any time if you are showing symptoms which is what the testing should only be used for IMO.
Interesting assumption to make. Did you know that 83% of over 18s in the UK have had the first dose and we are approaching 75% being double vaccinated by the end of August?
I guess I'm in the overwhelming majority of those in the UK who maybe can't have kids now or if I do they will have side effects.