This is only the case when you have something to fear from superior competition. That is the capitalist system. Even in the FOSS world, which is largely affected by capitalism the spirit of sharing and collective advancement is incredibly common.
Let me rephrase my statement, because I didn't make my main idea clear enough:
"When the competition starts using your own improvements/innovations against your products, and they are not moving a single finger to push innovation, you are obliged to keep the secret."
I don't mind if the competition improves my work, this is more than enough incentive to push innovation forward, but when the competition stalls and makes a verbatim implementation of my work, or worse enough, depends solely on my improvements to stay relevant, I honestly prefer to keep the secret to myself. I don't want parasite competitors, I want talented competitors.