I think that's too broad of a generalization. I happen to walk the line between designer and developer and I know a lot of my fellow designers are very open about their process and work. Designers often share their tools and techniques openly. See the multitude of CSS frameworks, free fonts, PSDs, etc. I think the notion that they're more secretive less open comes from these cases where a designer
does make a stink about "theft", "copying", or whatever the terms they use may be. The thing about that is, well, first design is a very personal thing. Yes, there are rules and best practices, tools, and techniques that apply universally but at the same time good design also carries with it a piece of the designer. We take our designs personally. It's very hard to release an open source piece of design work because you need to make it in such a way that it can be effectively customized or used in such a way that the larger context makes it a unique work unless of course it's a rare case of the designer not caring if everyone in the world uses the design verbatim (see: Wordpress themes).
I know for me, personally, I've put many design projects on GitHub - from the mockups to assets to the code to put a site together - knowing there's a chance someone will use it verbatim but hoping it gets used as a jumping off point or customized to make it someone else's.
With design, you can see when someone has ripped you off very obviously most of the time. You can't own design elements of course but its a very subjective thing that you just know when you see it. Developers and designers also think very differently in some areas. With developers everything is logical and black and white. With designers, much of their work is vague, fluid, and incredibly subjective. Emotion plays a role in the success of design often times whereas a developer's code will be just as useful and functional no matter how anyone feels about it. As a developer, when I open source something, I know that I can't own the concept of a loop or a database query or whatever. What makes my code unique is the way in which I solve my problem and how it solves a problem that other code has not solved. I'd be happy if someone forked my code, made it better, and started a new project from it that became popular. However I'd be upset if someone took my design, added on to it, and passed it off as their own. The difference? On the surface there shouldn't be one. But beyond the surface it's all about the piece of yourself you put into design work.
Hopefully this makes sense and doesn't just sound like rambling. I'm sure I may need to clarify a few things I said so please ask if I said something vague.