Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. We can disprove flat-Earthers’ claim by setting up two different weights of the same material (thus, the same density) against each other using a balance. We can see the objects’ densities are not why they fall down.
Human senses have limitations and cannot detect beyond a certain threshold of external stimuli. However, we humans possess intelligence that enables us to create instruments and allow us to perceive beyond the limitations of our senses.
Flat-Earthers argue if our senses cannot perceive a thing, then it does not exist. In reality, the instruments we made enable us to detect many things that our senses cannot.