Why do you believe this?
1.) The inherent mediocrity in large bureaucracies. They always turn mediocre. People inherently form factions within corporations and become politicians. This destroys the old culture and creates adverse incentives.
2.) Technology is always evolving at a massive pace. Old infrastructure in a large company becomes a burden, and stifles innovation. Startups are built to seize opportunities with new technology.
3.) Which tech companies that have historically been dominant since the 80s are still dominant? Intel? IBM? AOL? Yahoo? Microsoft (maybe but still somewhat irrelevant). Apple may be the only company, but they are in a different game.