Why are EXTRINSIC REWARDS inherently RISKY?

The following self help and motivational insight has been adopted from Daniel Pink’s book Drive

“The problem with making an extrinsic reward the only destination that matters is that some people will choose the quickest route there, even if it means taking the low road. Indeed, most of the scandals and misbehavior that have seemed endemic to modern life involve shortcuts.”

Thus making extrinsic rewards the only destination comes at a very heavy price.