Because the only ideas you can have are founded on what you already know.
You can’t dream up things that don’t exist if you don’t already know about the pieces required to make them up.
This explains why ideas rely on a certain state of culture and readily-available resources to come to fruition. The Internet wasn’t invented in the 1800s because the sum of it’s parts didn’t exist yet. Similarly, the iPhone, war drones, flower delivery services, the Nintendo Wii, and your grandparent’s famous oatmeal cookie recipe, all weren’t created before their time.
What you know is more powerful than your imagination, but imagination and having a lot of existing ideas is what makes it possible to have really big ideas.
Two great quotes come to mind on the subject, both from Steven Johnson’s 2010 book Where Good Ideas Come From
The trick to having good ideas is not to sit around in glorious isolation and try to think big thoughts. The trick is to get more parts on the table.
So yes, the best ideas you have are the ones you’ve already had, but you can get more by consuming more. Read more, walk more, talk to strangers more, click on weird links, share your ideas with others and invite them to share their ideas with you.
If you want to really have good ideas you’re going to have to consume and experiment occasionally too. Johnson’s second quote reminds us of this:
The patterns are simple, but followed together, they make for a whole that is wiser than the sum of its parts. Go for a walk; cultivate hunches; write everything down, but keep your folders messy; embrace serendipity; make generative mistakes; take on multiple hobbies; frequent coffeehouses and other liquid networks; follow the links; let others build on your ideas; borrow, recycle; reinvent. Build a tangled bank.