Musicians make music with the technology available.
Do you think if Beethoven were alive today that he would scoff at the intricate variations in sound possible with modern equipment, and stick with piano music only? I don't think so. The same applies to the symphony composers you mention.
Imagine creating beautiful music in a stream of pure ideal creation. Now imagine having nothing available to make physical sounds with except for different sizes and shapes of sticks. Could you still make beautiful music? Of course you could. It would just all be made with sticks.
If you were then given a variety of instruments to choose from, would you still use only sticks to make music? Maybe, but I doubt it.
Of course some people would continue to make music only with sticks, but note that they would be choosing to use sticks, rather than using sticks because they have nothing else.
Several hundred years ago, there was a far more limited range of sounds available from the musical instruments of the time. Music written for orchestras had the great advantage (over any single instrument) of making it possible to combine such a range of sounds in a single aesthetic piece of art. In other words, if you wanted a wide range of sounds in your music, you couldn't write for a single instrument.
Today, people still do write symphonies for traditional orchestras, but there is a wider range of choice. To simply create music with a wide range of sounds, it is not necessary to involve an actual orchestra. So people choose other things.
You're not looking at any strange disability in modern composers; as others have pointed out there are composers writing symphonies today. What you're looking at is an increase in the number of choices available to musicians.