Because they're huge and every decision has to go through many layers of approvals. In my company we took 3 years for a project I wanted to work on took off because it had to get the right Project Manager on board, with the right Technical Manager to buy in the idea and the right Head of Product to sign off on it. And with all that perfect storm, it took 2 years in between pitches and meetings to finally get the green light to start it.
Huge companies can be an equally huge blocker to innovation.