Its the viral load, or the amount of virus, mainly. Most people have a large viral load when they are the most infectious, especially in the nose and throat. Testing daily would ensure that you know immediately when you're highly infectious.
A recent check of the rapid test used in the UK is that its largely unaware of the virus if the necessary PCR cycles are > 25, gets it right half of the time for CT between 20 and 25 and it gets very good if viral load is large enough to be detected by < 20 cycles. Other rapid tests are AFAIK somewhat better.
The main problem that most (western) countries failed to solve is to remove reasons to not take a test and not to self-isolate, and that's primarily inability to get proper support WRT work / monetary situation while self-isolating.
Make it easier for people to self-isolate than not to, and people will do it.
A recent check of the rapid test used in the UK is that its largely unaware of the virus if the necessary PCR cycles are > 25, gets it right half of the time for CT between 20 and 25 and it gets very good if viral load is large enough to be detected by < 20 cycles. Other rapid tests are AFAIK somewhat better.
The main problem that most (western) countries failed to solve is to remove reasons to not take a test and not to self-isolate, and that's primarily inability to get proper support WRT work / monetary situation while self-isolating.
Make it easier for people to self-isolate than not to, and people will do it.