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.
Like many white suburban kids in the 1980s/1990s, I had a telescope. It was, at one time, as trendy as a rubic's cube. It was large enough that Jupiter was a brownish blob, with stripes of darker and lighter brown with a reddish blob. The reddish blob changed position.
Sometimes the image was more clear. I wasn't savvy enough in the preinternet days to know when Jupiter was at its closest approach. If it was higher in the sky you had a better view, less atmosphere.