Good grades are also an indicator of being able to optimize for grades. Being able to optimize a metric is a good thing, but the problems is that typical students spend 20 years learning to optimize that one and only metric. Therefore it's no surprise out-of-school students are often only good at optimizing similar metrics, and suck at the general creative + critical rational thinking that's required in life.
(I think I'm basically paraphrasing another PG article here, but this opinion mostly comes from my own experience working with both people from higher education and early drop-outs)
Good grades aren't the goal. Good grades are an indicator other people can use to gauge your preparedness for a job or opportunity.