A resposta mais simples é que aprendemos (no filme Minions) que foi assim que evoluíram. O peixe tem que nadar, os pássaros precisam voar, os lacaios devem servir um mestre do mal.
O romance júnior licenciado explica muito bem:
No que diz respeito ao seu último ponto sobre os lacaios que servem alguém "legal", você pode querer notar que, embora os lacaios geralmente sejam jogados como engraçados / sem noção, eles não demonstram escrúpulos em cometer alguns atos criminosos tão hediondos como roubo, assalto , dirigindo e voando sem licença, possuindo explosivos controlados e auxiliando e encorajando o roubo de um corpo planetário.Minions have been on this earth since the beginning of time. At first, they were just shapeless, single-celled organisms searching for the biggest, baddest villain to serve. They’d follow anyone despicable enough — as they needed to be led by something bigger than themselves.
Over hundreds of years, the Minions evolved, growing arms and legs. As they evolved, so did their choice of leader. Each master was eventually replaced—or eaten—by a bigger, badder master. At one point, they followed the biggest amoeba—until he was devoured by an evil fish who, in turn, was eaten by an evil amphibian. They followed that evil lizard right out of the prehistoric sludge and onto a beach… … where he was smashed by a Tyrannosaurus rex.
The Minions were very impressed. They stared in awe at the giant T. rex, with his terrible claws and teeth. He was the greatest monster they had ever seen—so much bigger and tougher than their other master. It was love at first sight.