The dose makes the poison. Just because you can detect the virus doesn’t mean it is around in quantities that can make you sick. I believe the CDC has updated their guidance to reflect this fact.
A single virus probably can cause infection, but it appears not to be anywhere near a linear correlation between dose and risk. And severity can be tied to initial dose, too.
This is one interesting thing about masks: they may do more to improve prognosis than reduce infection.
Further, when we're talking about ... e.g virus on cardboard... the way you do the test is after the droplet on the cardboard has dried, you use a buffer to dissolve the virus back out of the cardboard and see if you can grow it in culture. This may recover virus in cases where there's absolutely no chance for casual contact to get it out.