Because knowledge makes money. If others know what you have, it makes you less valuable in the eyes of management.
I see that a lot in my line of work, because on the one hand, shared knowledge is essential and on the other hand breakthroughs make serious money. I had old mentor talk about a skill she learned once. I asked her if she would be able to teach me - and I was willing to pay for it because I wasn't asking for free - and she still ignored me. There are a ton of idea thieves too out there. My current supervisor did that to me, and while I think she is OK as a person, she doesn't get
out of me as far as ideas go. Both my supervisors at the current job did this and because I sensed they were idea thieves, the ones I gave on the surface looked good, but if you're the discerning type you'd see that you shouldn't use them as is or you look like an ***.
Shouldn't have to do this, but you have to do this in 'Murica as original innovation is king.