Again, you can make similar arguments with alcohol, yet that does not prevent us from having blood level limits. As a person who drinks rarely, I'm majorly impaired well below the legal limit.
Alcohol and THC both diminish over time, as opposed to vanishing in an instant, so for both they can be detectable after a person is no longer intoxicated. They both affect different people in different ways and at different rates. If you want to make an argument that THC is not equally well/poorly monitored by using some sort of blood level analysis, you ned to offer a significant difference.