South Korea is far from the only place COVID-19 is present, and most of the rest of the world tests barely anyone, even if they have symptoms and would like a test, either due to lack of resources or formal policies of only testing hospitalised cases. I can detect my infection, but statistics on the rate it is spreading in the UK can't.

It's also pretty well established that even in symptomatic cases where testing is widespread the disease has a multi-day incubation period.