Well, you’d have to get close to it for one, and at that point it’s quicker to bash it down than to wait for it to burn. You also would generally need significant kindling to get the fire going strong enough to burn down something thick like a gatehouse door, and the defenders could put the fire out if you let up the attack at any point. Battering ram is just the easier option.
This picture got me wondering why not burn the big wooden door down?
Well, you’d have to get close to it for one, and at that point it’s quicker to bash it down than to wait for it to burn. You also would generally need significant kindling to get the fire going strong enough to burn down something thick like a gatehouse door, and the defenders could put the fire out if you let up the attack at any point. Battering ram is just the easier option.