Why human cells are used for drug testing?

I believe that the tissues being used to test for drug usage would be the fatty tissues. Even after the time elaspe period such as 2 weeks and then you can present a clean test, well not necessarily. If test are extensively done then drugs can be detected through some pathological test to see if it is stored in the fatty tissues. Even after 1 year of non drug abuse there could still be some detection, but lessening, and over the period of time clear.