Why does the AI attack with one ship at a time?

Let's say for example I have a single BattleAxe defending a planet. Many times I've seen the AI throw corvette after corvette at it. A corvette might arrive every month for example. They each get destroyed, thereby building up the experience and effectiveness of my BattleAxe, and it's basically fully repaired by the time the next one arrives. Doesn't that sound ... well ... dumb? Shouldn't the AI build up a force of 10 Corvettes (or anything better it might have) and then attack with the stack? At least it would have a chance of beating the BattleAxe ...