Why does it seem unreasonable to suppose that? The space of possible music is not quite Hilbert-space huge, but it's really, really huge.

So, to produce something like Beethoven, you have to be aiming rather specifically for that.

Very few composers frequently go into another composer's space and produce great music there. John Williams comes to mind as a good candidate, but he's not quite Beethoven-level. Why don't they? Novelty-seeking is an excessive explanation. There is plenty of good stuff left in those veins, but by going there, you're putting yourself directly up against Beethoven. There is somewhere you could go where you would stand out more. The obvious exception is when you're trying to fit a particular space due to a program that you didn't set (which handily explains Williams).

Once musicians have saturated music-space somewhat, you won't need a specific reason to returning to these spaces. As noted above, that could be a while.