The existing ideas are usually as good or better than mine, making it impossible to break in. Or if it's too similar, there could be patent issues. I'll give you two examples from when I was a kid.
I saw one of those geometric balls that expand and contract at the gift shop of the Smithsonian. I thought, what if they use something like that in a transmission instead of multiple gears? You could have one like that which expands and contracts with a smaller one that varies much less but just changes the pitch of the teeth to fit the big one. The next decade they came out with CVTs which objectively were a better design using (very basically) two rollers that change geometry.
In middle school, I created an invention for that young inventor program run by Sears. I made a device that would slide up and down on the side of an extension ladder. It was a basket to hold paint or tools. I couldn't find anything like it back then. 20 years later and I see an infomercial one. Perhaps this was my best idea I ever had. Sadly I was just a kid and never patented or sold them. I didn't even win the competition.