Why don't they do this before giving you a job offer?

Drug test usually take 1 to 2 weeks to get tested considering they have to send it to the lab and the amount of drug test they have to do. Also, businesses usually ume that people applying for a job will not be on any drug at the time. They ume that people are smart enough to get clean first. They mainly have the drug test to look good because they don't want their company name tarnish by them hiring druggies.