Here's a picture of a billiard ball under a microscope. We see it as really smooth, but magnified it actually has mountains and valleys. If the earth was shrunk to the size of a billiard ball, and you could hold it, it would be much smoother than a billiard ball.

So, okay, at the scale we interact with it, it has mountains and valleys and flat spots. Even so, it's pretty easy to see the curvature of the sphere. When you're at a high cruising altitude on a plane and look out at the horizon, it's curved. When you stand on the shore and watch a boat sail away, it starts looking like it's sinking as it goes over the horizon, over the curve.

It's a sphere, it's just (to us) a really big sphere.