Why is the electric field between ideal large parallel plates often treated as uniform?

Because conducting plates eliminate all charge variation in the region between them, forcing $\overrightarrow{E}$ to vanish

Because the potential difference is zero between equipotential surfaces

Because Gauss's law requires the field inside any capacitor to be constant everywhere

Because the plate separation is small compared with plate dimensions, field lines are nearly parallel and the magnitude is approximately constant away from edges