

BAC is also well correlated with impairment. Obviously it varies from one individual to another, but it is related strongly related enough to have fair and consistent enforcement.
AFAIK, blood tests that measure the presence of marijuana are relatively cheap, but measuring the concentration is slightly more difficult and is not well correlated with impairment. That means enforcement is problematic and subjective.
I probably overstated by saying it is equally dangerous with somebody driving drunk. However, there are lots of studies that show it causes serious impairment.
https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2788264
https://pmc.ncbi.nlm.nih.gov/articles/PMC9940647/
https://www.nhtsa.gov/sites/nhtsa.gov/files/documents/812440-marijuana-impaired-driving-report-to-congress.pdf
https://jcannabisresearch.biomedcentral.com/articles/10.1186/s42238-023-00202-y