Yeah and on bottled water which has filtered through mountain rocks for hundereds of years to end up in a overpriced plastic container for yuppies to buy.
serious answer...Because sour cream is soured by adding a specific bacillus (similar to making cheese) while excluding others. If left for too long, then other wrigglies come along to join the party, fementing the cream, making it nice and fizzy and growing it a nice furry coat.