If Umpires have a better vantage point from the chair...

[smoothasmecir]

why don't they elevate all line judes so they can have a better vantage point as well?

[ProStaffTour90]

because they don't have the whole court to watch, they can just concentrate on the one line!

An umpire can make over rules on any part of the court, even though it is sort of an unwritten rule that they don't make overrules on the opposite side of the court as they are still too far away!

[Max G.]

Because the line judges see better from down low, because they need to watch a small area. The chair ump needs to see the entire court - and so needs to be elevated.