Because your test is only as good as the moment you took it.
They're saying it's airborne, right?
Your test renders negative, you walk into a section where it's airborne, now its on your hair, you touch your hair, then you touch your eyes or maybe your mask wasn't on tight enough...
You go home, you think you're fine you didn't wash your hair when you got in ...You are asymptomatic, but you infect other people, your family members get infected now they have to do testing and you have to do testing again and so it goes on and on and on and who is going to pay for that?
Who's going to have the resources to test you multiple times over, and test your next door neighbour, your family members, your uncle your aunties, the people down the street.. every last person in this country. Please Mr. Internet Know it All, tell us how it's going to work.