But you don't have to do it, if somebody really wants to place 40 barrels and blow up the goblin army why not. If you do not want that in your game, then don't do it. Why would it matter?
There are a couple of issues:
1) If you -can- do it, they have to balance the game around the assumption that people might do it. So you might end up with a game that's either way too easy for people that set battlefields up with barrels or way too hard for people who don't.
2) I just find the existence of exploding barrels super annoying, as a small part of why I find all of the elemental surface stuff, grenades, magic arrows, spell scrolls, etc. really annoying. It's a way for anyone to steal the thunder of people who spend their lives training to make big explosions. What's the point of my wizard learning to cast fireball if Joe commoner can chuck a barrel onto a lit candle? And he doesn't even need to use a spell slot to do it! I'm the guy who would sneak onto the battlefield ahead of time just to remove all of the barrels before the fight.
If they want to set up a fight where the barrels play an important role, then fine, have them there. I'm cool with it if it's an intentional choice about how they are telling the story. But to just have hundreds of barrels of (probably very expensive) explosive materials just abandoned all over the world feels lazy and unbalancing and it detracts from what makes our characters special.