To be fair, that is somewhat the current scientific consensus on gravity. It just is. We can meassure it and determine a general constant and calculate with it (and even though some people claim to have understood way more, it is highly debated terrain as far as I know)

And in general, I was actually arguing with flat earthers recently a lot, I even met a flat earther in real life. It is an interesting intellectual challenge debating them. Basically rebasing all the physical theory I have. (Main summary is, they have a high ego, but lack understanding of everything and make up for it with make believe.)

If I found a school one day, one of the lessons will be the teacher telling the students: "The earth is flat! Proof me otherwise." Or more advanced, model a flat earth on a computer. Flat earthers try that for real - it gets weird very quickly, so much that I could not believe anyone taking it serious and it all is just satire. But they are for real (but with a very different concept of reality).