This is what origin is talking about. Your question is so clearly, fundamentally misinformed that it's actually hard to answer. If you did some math and made an algebra error, I could point it out. If you fell prey to one of the common misconceptions in physics, I could help identify and correct your error. In general, if you get to an incorrect conclusion via faulty reasoning, I can explain the errors in your reasoning. But I can't see any reasoning that would lead you to the idea that a person's inertia is always pointing toward the Earth's surface, so I can't explain why it's wrong. All I can say is "that makes no sense."
To extend the computer analogy, let's say the computer expert gave a quick summary of how programming works, hoping to correct the theorist's faulty thinking. The theorist responds: "But if the computer we're trying to program has a good web browser, why can't it just parse the html?" The expert could go back and try to re-explain how programming actually works, but the only direct answer to the theorist's question would be "I have no clue where that idea is even coming from." Your question is similar: I could go back and try to explain the vector nature of momentum again, but the short answer is "I can't think of any reason why you'd expect your inertia to point towards the surface."