I think you have it backward. Developers aren't born knowing how to keep design simple. Left to their own devices, most developers will create a tangled, incomprehensible mess. Design principles provide a set of guidelines that help give guard rails to developers to be able to produce simple, or at least somewhat intelligible, code.

The sad truth is that most developers never develop to the point of being able to create intelligible code, much less simple code. Really, in any industry the number of people who are truly good is vanishingly small, and our industry is no exception. Only the very best developers are able to break out a problem so it's truly simple.