If it's obvious to anyone with a telescope, that at least Jupiter is a sphere, why would Earth be flat?
One answer I hear from flat earthers is that other planets are round, but not Earth.
The other answer is that space is some sort of projection, and you're not seeing planets are all.
Many believe that the ISS is a result of project blue beam when viewed from earth, as it has a holographic appearance. It's a fact that there are satellites floating high up in the atmosphere (sometimes in jet streams) suspended by advanced balloons, not in orbit in the vacuum of space. On a flat plane long distance communication would be easy with repeaters set up. We have had land based navigation systems like Loran since the early 1900s that the curve of the earth does not seem to inhibit at all, maybe cause there is no curve.