The reason is simple. We haven't co-evolved with lab-grown food for several billion years, or had transmitted cultural experience with it for tens of thousands of years. The recent discovery that artificial sweeteners have unforeseen health effects (hyper-clotting / glucose intolerance) should point out the necessity of caution.

The human organism has been in a very complex dance with its environment for millennia. We have no idea what subtle chemical traces it is grown to depend upon. We pay for our arrogance and never seem to learn.