Why did they stop being funny?
I was struck by several of the ideas in b9's post in combination with someone mentioning mathematicians and physicists; because maybe the thing they have in common is making connections between apparently unconnected things. This can produce humour, or a new physics/maths theory.
And yes, maybe it's something that dies off with age. So that they retain all the body movements/grimaces/wiggles etc, or the interest/obsession in symbols, but without the creative connections to give them meaning, so that become embarrassing physical/facial or symbolic contortions without any content.
And the other thing could be grandiosity/prima donnarishness of some, and/or simply making bad decisions about films and directors, as several people have suggested. For instance Peter Sellers' "longevity" seems to have been the result of spending his money so fast he agreed to make more Panther films with Blake; which he apparently wouldn't have done otherwise because he thought that they were trash! As if the "childlike" quality which often leads to great humour also needs looking after/protection; they "don't know what's good for them"!
It was Steve Martin's slide which first "hurt", for me, then Eddie Murphy, and Dan Ackroyd, ( I never liked Robin Williams ), and then more recently Jim Carrey, ( except that he has redeemed himself with some brilliant serious films ), and a french comedian, Jose Garcia, who used to be brilliantly funny, ( see scene in morocccan hotel in "Le Boulet" ).
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