The Simpsons franchise generates massive revenue and returns for Fox. This is in no small part because of the "relationship" that viewers from across the globe have developed with the Simpsons characters, which are highly reliant on the well known voices. Harry Shearer does some very distinct voices for characters like Mr. Burns and Ned Flanders. His services are therefore involved in far more than mere talking as you suggest. He adds an immense amount of value and deserves to be well compensated.
This is the same reason other actors, professional sports people, famous musicians etc. make the kind of money they do. They add value ... Besides, $6,000,000 is a lot less than many people at the top of their profession in the corporate world make.
I'm not saying he's a good/bad guy or that he should be paid less/more. All I'm saying is that I would expect someone who is as central to the value creation of the Simpsons to be extremely well paid and think it is entirely justified.