I think part of the reason is also that tech has matured to the point where it's so complex that nobody can handle it alone or even in a small group. A couple of people (no matter how supersmart) simply could not build and maintain even a single product of a large tech company.
Similar stuff happened with other technology too. A couple people could build an early airplane but a modern jetliner is so tremendously complex that you need years and a full company to get one out the door.