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Because grad school is about research and nothing else, and you need to know that you love research enough to do it 60 hours a week for five or more years. Grad schools want to know that you’ll be able to get yourself through your PhD program, and people who are poorly prepared in terms of research experience tend to drop out of PhD programs at higher rates.</p>
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I think it’s worth noting that this isn’t just all about power – the professor is probably not in the lab very much, if at all, and the grad students and postdocs are the ones on the ground who know where all the materials are and how to do the protocols. 
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<p>If you asked my faculty advisor where to find the chemicals or how to run one of our lab’s machines, he’d stare at you like you had three heads. He hasn’t done bench work in twenty years!</p>