There's a time frame between tests, so they can contract and spread anything between that period. As slutty as they are who knows where they have been and who they have been with between the period of their last test and their next test.

What if the chesmist fucks up during analysis? For whatever reason. What if they misdiagnose? It happens more often than people think. That's a woops that no malpractice suit will ever fix.

Also, not all studios require tests. It's an at your own risk setup for most. Especially amature productions.