Again, think of fire. You could ‘test’ ie. observe for fire in the middle of the Fire of London, but the results wouldn’t tell you anything that wasn’t already obvious.
Conversely, when there isn’t widespread fire, observing for outbreaks is essential precaution, and highly practical because it enables you to extinguish them before they spread.
For trace-and-test to be effective, you need far more tests than infections, and the majority of those tests need to be well targeted. That is achievable when there are only a handful of new infections per day, as China and Australia are demonstrating. The UK has never achieved that enviable position. We were fairly close last summer, but Boris chose to give 37 billion to his friends while pretending to spend it on trace-and-test.