Nobody writes in that style of music anymore. It's "already been done". Even if someone wrote something as good as Beethoven's 5th, nobody would care. The classical music world has been there, done that. Mozart, Beethoven and Mahler aren't remembered just for their music on its own, but the fact that their music pushed the envelope, invented new forms of expression. They didn't just do it, they came up with it.

Classical composition moved in a kind of academic, atonal direction in the 20th century which was completely inaccessible to most of the public, and essentially died, in terms of a popular artistic movement. After Stravinsky, there just wasn't really much left to do. (Composers still write movie scores, but these basically just rehash what was already done in the late 19th and early 20th centuries.)

Instead, the world moved towards rock, pop, hip-hop... and there's a lot of exciting stuff there. Unfortunately, it's mostly based around 5-min songs, which limits its complexity. No matter how genius something like Brian Burton's "The Gray Album" might be, or certain Led Zeppelin songs, the format doesn't allow for even 5% of the kind of complexity that a symphony can embody.

So the Bach's, Mozart's and Beethoven's are surely out there -- but they're not doing the same kind of music, and they're harder to recognize as equivalents because we've abandoned musical structural complexity for sonic complexity instead (it's more about the sound or flavor or texture of a track, its hooks, and so on).