Why does mineral water that 'has trickled through mountains for
Centuries have a 'use by' date?

Who was the first person to look at a cow and say, 'I think I'll squeeze these dangly things here and drink whatever comes out'?

Put a closed bottle to a bright, warm place (especially if the bottle is a plastic one), leave it there for three months and try it afterwards. It might taste ugly. Perhaps you can also see some sea weed (not very probable).
Bottles are not necessarily sterile.
And plastic bottles cause bad taste rather quickly.

Why do people have a need to answer rhetorical questions? (sorry, could not resist....) :p

Was that so rhetorical??? Then forget the answer and continue wonderting and smiling :D