the aging process occurs in the barrels because of the organic nature of the wood that they're aged in. this releases flavors and other chemicals as well as absorbing some of the "nasties" that will cause fresh whiskey to have it's "sharpness" tha makes it undesirable. in addition, the air where it's aging can still seep into the barrels and help impart flavors, which is why genuine scotch whiskey will have a distinct taste to it because some of the flavors of Scotland are imparted to the drink. once bottled (granted it's properly sealed and stored in shade) the aging process will effectively stop because glass isn't permeable.
beer and wine however, have nature organic compounds suspended in the liquid, which allow the conversion of sugars to alcohol until it's effectively "used up." i have some 3+ year old bottles of Dogfish Head 120-Minute that reach their peak aging at around 4 years. they are bottled at 18% alcohol and will reach upwards of 25% after 4 years are so. there's SO much fruits and natural sugars used to brew the beer that after awhile aging, the flavors start merging together and smoothing out because of this process. i've had beer ranging from fresh out of the fermentation tanks to 7 year aging and everything in between and can tell you this happens for this reason.
hope this helps!