Quite sincerely, I'm convinced YC and other VCs don't do this because they've observed that ideas truly don't matter very much. And if you can't even come up with an idea what hope do you have of finishing the work, why not become an employee (which is fine)?
What matters is the journey and how you react to the multiple problems along the way (99% of people give up too early, too much grit is required). Usually by the end of that journey your initial idea has changed beyond recognition, even if there are some common elements between the idea and the endpoint.
To come up with some examples:
Apple: Computers for everyone -> Phones for everyone
if ideas don't matter, explain Uber and the dozens of copy cat apps that launched right after all over the world. Why didn't they launch before Uber was even thought of by the founder?
What matters is the journey and how you react to the multiple problems along the way (99% of people give up too early, too much grit is required). Usually by the end of that journey your initial idea has changed beyond recognition, even if there are some common elements between the idea and the endpoint.
To come up with some examples:
Apple: Computers for everyone -> Phones for everyone
Amazon: Buy books online -> Logistics, Free Delivery + Cloud Compute
Airbnb: Airbeds in living rooms -> Online bookings
SpaceX: Greenhouse on Mars -> Reusable rockets
Very few ideas survive contact with reality unscathed.