"I just can't fathom the idea of a flat Earth and why someone would think it's conceivable."

Because of all the stupidity involved.

" If the Earth were flat what would keep us grounded? "

Buoyancy, of course. (Aka, stupidity)

"What happens when you sail to the edge? "

You can't, because all world govt's have provided a fleet of navy ships that patrol the icewall that surrounds the edge of the flat earth disc. (aka, stupidity x 1,000)

But if you did make it to the ice wall and continued, you'd either hit the invisible dome (that holds the stars/moon/sun) or you'd just keep going out into the infinite plane of the flat earth, depending on what idiot you talk to.