Along with what baybal2 says today they'd have to fit in to a corporate environment that rewards credentials over demonstrated skills, gladhanding and backslapping over geeking, financial engineering over the creation of tangible things.
Well you also have to remember that companies came and went faster than startups did. And consolidation happened quite quickly as well. Engineers made cool things but only a handful of the companies found a hit, and rarely did the same company consistently find the best. What ended up happening is the bigger companies just started buying the small company that had the hit that year. The old model was no more sustainable than the current model.
