Why does it seem that all the great classical composers are dead?

I think you need to do a little more research and listening before making such a rash statement.

There are HUNDREDS of composers alive and thriving today. Among those that could be considered truly 'great', or whom I tip for greatness in the future, include:

John Adams

Thomas Adès

Kalevi Aho

Elliott Carter

John Corigliano

Henri Dutilleux

Brian Ferneyhough

Sofia Gubaidulina

Helmut Lachenmann

Per Nørgård

Arvo Pärt

Wolfgang Rihm

Poul Ruders

EDIT: Eric: there are some quite young composers (I know of some in their 20s, 30s and 40s). People who will eventually become classical composers are being born all the time. Or are you suggesting that one day the breed will have died out?

Expired, checked out, crossed over,

... Yup, Mozart, Beethoven, Schubert, Bizet, Puccini, Verdi, Tchaikovsky and Bach are all dead.

There are literary thousands of composer some from the time of Rachmaninoff that are still alive but they're old. And there are small groups of child composers from this century that are trying to become the next Beethoven, Bach, or Liszt. I myself am a composer. You just need to keep looking.

Edit.

On the contrary, I'm I know that there will be billions of thousands composers to come in the near future.

Well, being an early music fan, the earthly remains of all of my favourite composers are well and truly mouldering.

But to echo Del, there are plenty of fine composers who are very much alive. I especially like the Estonian composer Arvo Pärt.

Hafwen.

Aaron Copland died in 1990, is that close enough for you?

I think the majority of people who listen to classical music have kind of overlooked everything newer than the early 20th centurey for some reason. Maybe Schoenberg scared them.

Because the classical era of music is already gone...if you mean the original classical composers

Try John Mackey. He's great.

Because they are, in my perception (ALL DEAD).

So I'm an ole stick-in-the-mud,

Alberich

HANS ZIMMER CLINT MANSELL THEREI PLENTY!!!

have no clue