Out of Date?

So, what's the deal with use by dates on things that are staple like products?  I cleaned out my cabinet yesterday to get cans for my nieces can-a-thon at school and decided to check some of the stuff that had been in there for ages to see if it was still in date.  There were several of those microwave desserts in a bowl that I had bought to try that had been sitting there unused because we just generally don't "do" those things that had gone out of date.  This led me to think about how the food manufacturers determine how long something will last?  These food manufacturers put so many preservatives in these foods that you would think they would last forever.  Canned goods are supposed to be good for years but those seem to have an expiration date that doesn't last that long either.  What gives?