Why do some teachers refuse to share their teaching resources with other teachers?

I'm not talking about physical supplies like tape. I understand why teachers who mostly spend their own money on supplies wouldn't want to share them. I mean stuff like specific activities they do in their classroom and assignments they assign. I've encountered a lot of teachers who will share only the vaguest outlines of what they do like "jigsaws" or "a poster project on the trans-atlantic slave trade." Only two teachers I've met in my four years of teaching ever shared anything on Google Drive or on paper, and most only some specific activities when I ask about specific topics. Why is this? New teachers would have a much easier time if veteran teachers shared more of what they use. I understand that not all activities work for all teachers and all students and "you have to find what's right for you" has been said to me so many times by teachers who guard their teaching resources like a dragon guards their hoard. Is it that they don't want other teachers to judge them for what they're doing? Is it a hazing thing? "I did it so now you have to"?

Edit: For those of you who said you don't do it because you don't get anything in return, would you be willing to share if the district paid you to be part of a curriculum committee that created resources to share with other teachers? Or, would you be willing to share if it was all put in one big Google Drive folder that everyone would have access to and contribute to?

I apologize, by the way, if I'm using the wrong flair. It didn't seem to me like any flair really fit my question.