Because here is what VCs do: they forward some money to you, they sign the investment agreement. Done. They don't need to do any more work except get progress updates from you (basically they read a single email) once every couple months. If they're on the board they'll meet with you for a few hours every month. That is not a lot of work per startup.

A founder needs to put in at least 8 hours a day for a single startup.