Why do non-native English speakers sometimes omit articles?

Have you noticed this? You hear a guy with an accent speaking in English, and every now and then an article goes missing. I’ve seen (or, well, heard) it happen to people from different backgrounds (Mexican, Russian, German, etc.). Why does this happen? It’s not like we don’t know how to speak the language: whenever this happens to me (I’m a native Spanish speaker), I notice it right away, and it never happens when I’m writing instead of speaking.

My theory is that we focus on getting the most important words (i.e., the nouns) when we’re “translating” our thoughts, so sometimes an article gets lost in translation. What’s your theory?