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.
Just as one can go jogging but not run a marathon, one take take a partial view on mainstream science on a particular topic as correct or incorrect without taking the whole of mainstream science.