Barrel usage history
An interesting aspect of barrel use is the ancient history of it and the fact that newly made barrels have almost no difference from those used 500 years ago. It's a timeless art and an improvement over animal bladders or clay pots. As to the question why they don't just throw a chunk of charred oak into a bottle, I think the distillers would rather present their product as properly aged before sale. Also the foreign object might be a choking hazard or hindrance to pouring. Doubtful there are still cooperage unions now, although it would be a reasonable way to maintain continuity in this craft requiring highly specialized skills, similar to wood boat building.
Remember the bushel basket and fruit crate era? I do.