Most ideas are bad.
The people who generate the best ideas generate a lot of bad ones. In fact, the reason they come up with good ideas is because they generate so many total ideas. Only a few are good.
But even those ideas are rarely good to start with.
Ideas need work, just like a piece of writing.
They need feedback from others.
They need editing. The best parts need more emphasis, and the weak spots need trimming.
And like writing, so many people don’t do the work required to make a good idea great.
If you want to have great ideas, you have to do the work.