I've found that transparency comes with strength. When you have nothing to hide, it takes a lot of extra unnecessary effort to be opaque, so you might tend to lean towards openness.

When you're weak, you lean towards opaqueness and even deception (think Sun Tzu).

I think it's natural human behavior. So rather than asking "Why not be transparent?" I'd ask "Why align with a company that is opaque? Isn't that there way of saying they have something to hide?"