I think you overestimate most biology labs. It's really easy to screw up a protocol in subtle ways. Often it took the original lab weeks of debugging to get it to work (yes, to get it to work, not to produce a false positive).

Things get more reliable as time goes on and methods become better understood, but the critical steps in most experiments are usually nontrivial to replicate in the beginning. Especially by CROs, which are notorious black boxes, incredibly expensive, and often just plain old unreliable.