Why was Karl Marx never in ANY marx brothers movie?

Because his face wasn't 'screen worthy'.

He actually did the voice for Harpo....

Karl Marx Brothers

I'm sure you can find some Karl Marx in the Marx brothers film "Duck Soup" if you look hard enough.

Karl Marx died in 1883, before any of the Marx brothers were born. Although the The Communist Manifesto, is a good laugh Karl just couldn't do standup.

Karl marx was not one of the marx brothers

The reason was it was a toss up between Zeppo, Gummo,and Karl. Groucho, who always had the final saying decided to put Zeppo and Gummo in the group instead of Karl. In other words, Groucho had betrayed Karl, but it benefited Karl. This lead Karl into being a political theorist, sociologist, philosopher, and communist revolutionary.

KARL Marx was NOT one of the 4 brothers making up the entertainment family. They were Harpo, Groucho, Zeppo, and Chico. Karl Marx was a 19th century philosopher who wrote THE COMMUNIST MANIFESTO, the so-called "bible" of that school of thought.

His Englisch was nott goot enough.