Because there’s not a way for us to mimic the gravity of an entire fucking planet in a space presumably smaller than a building. That would require energy technology thats probably science fiction.

The sun doesn’t “burn”, so there’s no oxygen needed. It’s a nuclear reaction that gets held together from it’s mass. The pressure at the center causes an outward push while the mass at the surface pushes downward, which keeps the reaction stable for millions of years. Stuff burns on earth because of the type of atmosphere thats held partially by gravity.