[epgui]

That's not at all what engineering is. Engineering is the application of science (including mathematics) to solve technical problems and/or optimize processes.

An engineer also has to eat lunch in order to function, but engineering is not the same thing as professional food critiquing.

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To answer your question, it's pejorative because politics is about allocating resources one way or another, towards and away from particular social groups. It's inherently going to be making some of the people unhappy, 100% of the time. Real life politics also tends to not be done smartly and honestly at all times and in all places, and this can make some people upset as well.

Most people associate all of that together, and most people don't take an academic (or particularly analytical) approach to the subject.