This wasn’t at all obvious. Hold a pencil out with your hand. Let go. Did the pencil just stay there in midair?
The fact that things fall because earth imparts an invisible force on everything around us, and that in fact, every massive object imparts that same invisible force on everything else around it, was pretty revolutionary.
It’s hard to overstate how unintuitive this would be to someone who had grown up at that time. Believing that something sufficiently far from earth would just float around and not fall was probably too much for most people at the time.