There is nothing wrong with the conventional wisdom "Keep it Simple, Stupid" apart from its apparently endless repetition. Try searching: there are countless books and websites preaching simplicity - from cooking to wildlife protection.
The repetition is what tells us that there is a problem. Clearly you don't make things simpler by simple fiat, although people often try to hide or suppress the real complexity.
What is missing from the conventional wisdom is an acknowledgement of what makes simplicity so hard. So let me try and assemble a few pointers.
Brad Bollenbach (Keeping It Simple, March 2008) identifies two primary reasons why we overcomplicate things. The first is a loss of focus, and the second is the imperial nature of the human ego. Leo Babauta (ZenHabits, February 2007) blames multitasking.
While these factors certainly explain some situations, others are caused by outright conflict between rival notions of simplicity. In engineering, we can see two rival camps - the agile/bricolage camp and the abstract/architecture camp - each of which believes that its approach leads to real simplicity while the other introduces unnecessary complication - the two rival camps only joining in their utter condemnation of a third camp - the baroque.
Scientists appeal to Occam's razor - the simpler the theory the better - and we are told that complicated theories such as Ptolemy's planetary models were replaced by the more straightforward theories of Copernicus and Galileo. The trouble with this version of events is that the new theories weren't actually more straightforward; they were considerably less accurate, at least until Newton worked out the mathematics.
Simplicity (and especially a fixed notion of what counts as simple) can easily become a fetish - something that generates its own complex.