Because we are humans.

Long answer, it is contrary to human nature. It would be very hard to get the richer to surrender their wealth to the poorer for no return.

PS: How did you get that socialism and communism aren't related? According to Marxist theory, the "evolution" of society is feudalism > capitalism > socialism > communism. I'm presuming were talking about Marxist communism, unless you would mind stating which other branch of communism is in question.