Because people forgot in a hanfdul of years (that's what it is, compared to the span of human history) what was slowly learned over the course of centuries and millennia. And the result? That kids nowadays almost think that foods magically pops out from the shelf at the supermarket.
Specialization (in the sense that nobody does everything, everyone does his small part) is definitely good, unless you forget what everything is about and depend on others too much.
Nobody expect people to go back to farming, but at least to know how is done. You never know.