They try to work hard and get rid of bias through various methods: peer reviews, precise methodology (randomization, blind tests etc.). And of course, even those sciences are not perfect, subject to biases, and evolve. This applies only to a subset of fields, in a lot of fields you either don't have the time to remove biases this way or it's impossible because it's inherently subjective. Besides, if you needed unbiased perfect opinions before taking any decision, the world would be stuck and slow.