Investors buy a cask of whisky when it is first produced and then hope that it rises in value as the spirit ages in the barrel.
It takes three years for spirit to become Scotch whisky in a cask, and investors are encouraged to keep barrels for up to 10 years or more to maximise returns.
It does evaporate over time. I worked in booze auctions for years. There are a lot of older bottles that have never been opened that are half full.