I like my tables to have hills and hedges, walls and buildings, streams and bridges. Ancient battlefields just look a bit, well, boring to my eye.
As has been mentioned above, ancient battlefields were generally quite sparse in terms of terrain. And when there were terrain features, they would generally be areas of rough or rocky ground, hills, maybe a vineyard, but not normally man-made features such as hedges, walls, buildings or bridges.
Actually I think many tournament type wargames rules that include terrain generation rules actually end up with more terrain that the typical ancient battlefield. It’s rare to see a completely open battlefield in a tournament game (as normally in these rules the players have some control over the terrain, and one player will want more terrain than the other) but many ancient battlefields were completely open.