If true, it's mainly because nCov can spread before showing any symptoms. And if true, that testing before 1 week after infection showing negative sign makes it worse.

Now imagine how to prevent a net virus from spreading, where it doesn't show any symptom and scanning it before 1 week shows negative result, while during that time it can easily infect other.

Testing in real people isn't cheap, and currently there is no proven cure either (afaik).

Vaccine is the best solution but it hasn't been found yet.