That is actually wrong. For a battle to occur, both sides need to agree to battle, except in rare circumstances. So, usually what happens is, that the general in a weaker position avoids the other guy, until he finds terrain suitable to his advantage. The other guy then of course is a lot less inclined to attack in for him unfavorable terrain. Compare that with a siege, the attacker knows where he needs to go, and what he wants to do, and for the defenders their fortifications are a lot better than moving somewhere else. (It should be mentioned, some armies could force a battle. The Mongols for example where just so much more mobile, that they could actually catch an enemy who tries to evade them.)

Consequently also in antiquity, most engagements where sieges rather than set piece battles.