Ask HN: Why don't Universities start/own more companies?

It occurred to me recently that Google/Apple/Amazon/IBM/etc. have armies of scientists that innovate, try to understand it fully, control it, and then design it to be used by the masses. Why don't universities like Stanford, Berkeley, Harvard, etc. use their tremendous funding (endowments + tuitions + licensing + etc.) to do the same? I understand that some currently do own at least part of some companies in some cases (I.e. Stanford with Google), but why don't they incubate and own companies entirely? Some certainly have both the business and science talent, wouldn't the next step be taking some risk and using their army of professors and business leaders to build and own companies in house?