Even snooker players will stand higher when the cueball is close the the object ball, except on maybe a very long pot.
I believe the transition from straight pool to rotation games has changed fundamentals. In straight pool, most of the pots are relatively short, from the bottom half of the table into the bottom corner pockets, and the cueball is frequently very close to the object ball. In this scenario a more upright stance may assist in sighting the ball. Also, fewer stroke shots were required per game, with most being center ball "punch" strokes that moved the cueball relatively small amounts. In 9-ball, long pots with accompanying large amounts of spin are often required, which are better served by a lower, longer stroke.