Actually, I'd say with no evidence, you'd be more likely to assume the earth is flat. It certainly doesn't look spherical from our level.
But the problem is, there has been shed loads of research done, by various organizations over the past hundred years or more, that shows the earth as round.
Without bandying "Conspiracy" around, there is much less evidence in favour of a flat earth. Most of it seems to be "it looks flat", a few relatively untested hypotheses to explain things like perspective and curvature, and some odd (but nonetheless clever) bendy light theories and equations.
(There's no doubt more, but I can't remember it...)
Round earth evidence is much more compelling. Examples include Continental drift and Plate Tectonics, Fossil evidence, the same rocks on different continents, Ocean conveyor systems, geophysical mapping of the internal structure of the earth, plus all the space programs and satellites. Again, there's probably more I forgot or couldn't be bothered to type out. A good old search will give you plenty.