Well, you can thank the 1990's for that. Folks were cramming to get Microsoft certifications but didn't know how to do the work and companies were hiring solely on credentials.
Then problems arose the "admins" didn't know how to handle and some companies crumbled. So the "fix" was to hire using contractors. Companies benefited because they were paying less in salaries, didn't have to provide insurance, and could "exchange" a worker if they didn't like them.
Then the trend moved away from just IT folks and everyone jumped onboard. Today about the only way you can get into a company is via a temp service.