It’s not just that you’re willing to let them have your idea; it’s also that you posted it on a public forum for everyone to see. There’s nothing stopping someone else as claiming it as their own and turning it into a legal landmine. Sure, you could prove it came from you, but that’s a legal mess whose untangling is just not worth it for most companies.
Not only that, but your “Well they can have them if they want them" probably doesn't cut it as a legal admission of donation. The only company I know that actively welcomed regular card idea submissions had players throwing in a legal boilerplate. I'm talking a complicated bunch of legalese just to ensure they were in the courtroom clear.
Don't get me wrong- I believe you're sincere in your idea and donation. As a long-time CCG player, I've had my own fair share of card ideas that I just wished a game company would look at. But being really careful when it comes to intellectual property matters is just practical for companies such as FFG.