Funny that you want full realism in mechanic you dont like ...
But not for the others. :-/
It's not just realism, but also a very obvious violation of the rules in the D&D universe. Even the strongest of adventurers is going to have ZERO chance of carrying around 10 barrels of explosive fluid equal to them in size and a half their weight in their backpack without any sort of assistance from magical means. To say that people are only arguing in favor of realism in a world with magic and monsters is disingenuous at best.
Beyond that, it's an issue of game mechanics. The commonality of explosive barrels and their like in this game basically shapes the game's default combat experience to be that of placing barrels and blowing them up to end an encounter as soon as it starts. It turns what would otherwise be a creative and proper RP solution for a difficult encounter into something you do for every big bad because there's so many barrels to spare.
