why does one say debajo de la mesa
"De" is a preposition, and there is no logic to these pesky little words. They cannot be directly translated, their use differs in every language. You've just got to learn when and which ones to use. It's the last thing you will learn to get right, and it's also the first thing you forget when you stop using them. It's hard for everybody. Just listen to non-native English speakers and take note of how often they it wrong when it comes to the use of "on, of, in, over, about, at," etc. That's because these do not correspond with how they are used in their own languages. So, it's only fair if you're having a hard time with it in Spanish ;-)
Good question....I'm a native speaker of Spanish, but I haven't really thought about that!