science - Why scientists don't share their research ideas?

A lot of scientist that I've talked to are worried about sharing their ideas with too many people. They get very protective of those ideas. Here are some reasons why:

1. Ideas are harder to define on your own


2. Ideas are hard to validate


3. There is no good way of getting constructive feedback outside of my network


4. Ideas get stolen with no reference or credit to the source


5. Afraid of stupid idea

As a scientist, do you agree or disagree with these observations?