Wheatstone bridge why not use a voltmeter?
The likely reason that the classic Wheatstone bridge is showing that a galvanometer is used to measure the bridge imbalance is that "back in the day" this was the most sensitive way to measure a very small bridge change. A galvanometer could be built that sensed very small current changes.

These days "in modern times" with circuit advances being made in semiconductor devices that have ability for input impedance levels up to the 100s if not 1000s of megohms it is possible to sense a change in the balance of a bridge without having to use the classic galvanometer mechanism. This allows bridges to be built with a lot more flexibility on the resistance ranges that can be used in the bridge elements. There are still practical limitations though. If a bridge is located some distance away from the sensing circuit and the bridge resistance is too high it can result in noise pickup from the environment which will can drive proper sensing into impracticality.