Placebo effect. CBD and THC are not well absorbed topically and there's no data on transdermal bioavailability, so the effective dosage would be unscientific and unknown. The majority of CBD creams fall within the category of snake oil to dupe consumers into spending money just as they do on beauty products. THC absorption by the skin is very low.
https://pubmed.ncbi.nlm.nih.gov/12100860/