Is it just bad luck?
In my recent England game my generals, admirals, conquistadores and explorers also dropped like flies. But, every 75-100 years I got a general that lasted longer than the others (like 15-25 years) increasing the average a bit.
Same story for my kings - most of them don't reign for longer than 7-15 years, only a few made it to 20 years.
Even though it's just part of the game, sometimes I think it's a bit frustrating.
PS. In my first EU3 game ever (as Portugal), I had a king (João II, I'll never forget

) who
reigned for about 110 years. So don't ask me how old he got! As he was not such a great king, I started to get desperate to get rid of him (but I never tried to make him fight impossible battles though).