OP is like a Trumpian argument from ignorance. “I don’t know anything about molecular biology, biochemistry, virology, medicine, germ warfare, or the history of pandemics, so please explain in simple terms how we don’t know this is man made.”
The answer for you is that informed people have looked into it and determined it wasn’t. The real answer is that there are common sequence (and other) markers for human manipulated organisms and this has none of them. Also, humans don’t know enough about genetics/biochemistry and immune response to knowingly create new viruses that have an intended effect. Best we can do is try a bunch and hope they don’t wipe out humanity, which is one reason it’s both stupid and an act of war and violates treaties to try.
I swear the Trumpian/Tucker Carlson/Joe Rogan, “I don’t know shit but I’m just asking questions” (wut about that 5G?) is the core of most simple minded thought. People who know about a domain see a blank page covered with 50 dots that don’t connect. People who don’t know anything see 3 dots and draw a line connecting them and think, “well, that’s revealing” (this isn’t only in science, it’s extremely common in peoples’ understanding of the forces that shape politics and society in every culture, eg, in middle east politics and society, brexit, really everywhere. Introduce religion or some bullshit about the masses or landlords or whatever and you are singing the song of the people in a language they understand.) The “secret” is that the leaders are also rubes themselves. They are also Believers.
This is basically Trump (or duturete, or bolsinaro, or Putin, etc). Be as stupid as the average person and have a party that is so ignorant and craven and grifting (gotta also attack “the media” for obvious reasons) that no one corrects and you can appeal directly to the people, especially if you demonize one group or another. Then you can do a lot based on whatever “plausible” delusions you exploit or sell and at least partly embrace.
So, to answer your question, we know because it’s the kind of thing Tucker Carlson and Trump would encourage (and themselves believe), and thus requires a bottomless well of ignorance and stupidity. (Not calling you out personally, but I’m triggered by the spectre of this sort of conjecture and there have been articles written on this at serious outlets like Vox, New Yorker, or whatever, not to mention academic sources, that go into the details).
It all goes back to Socrates’ response when he was informed that the Oracle at Delphi proclaimed him the wisest man in Athens. “I don’t understand why the Oracle would name me. I don’t know anything.”