1. They sometimes are. The imaging here uses a telescope. (Coronagraphs work on Earth-based telescopes too: https://en.wikipedia.org/wiki/Coronagraph.)

2. They don't need to be. Most telescopes aren't trying to image the corona of the sun or another star.

3. Space-based telescopes are really expensive and have significant limitations on size, shape, mass, etc.

Sorry I meant instead of having one big piece of metal which would have restriction in terms of length, having 2 pieces of lenses seperated by just vaccum? You would have lesser material, easier launch and higher magnification because of larger focal length etc?

Mostly because they don’t need to be, i would guess.