The More The Merrier

When I was younger I used to send ideas for games I had into the companies in the hopes that they might use them.Now that I am much older and realize that they have ideas coming in from all over.I recall Nintendo sending me a reply that stated that they have people they pay to do this.My question is,are they legally bound to use only the ideas their employees come up with,or is it simply a matter of not having the time to consider what must be a huge income of suggestions?

It's a legal issue. They don't want to get sued by someone who submitted an idea and claim that they stole the idea. It has to come from an internal source.

Not to mention, they probably throw all the ideas away instantly, after shredding them multiple times to make sure they can use it in the future.