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Because our literal world we're currently living on is built upon those studies and the model on how we review them. They are the building blocks for our modern society. There are thousands of studies being reviewed every week. 
It is only natural that some will be accepted only to be found faulty later on. That is just the nature of science.

And of course there is the economics and politics behind some studies that purposefully steer them to fit a narrative. But that is also just human nature. We cant just say they're all bullshit because of some rotten apples  ¯\_(ツ)_/¯