Why do today's musicians find some baroque and classical period music very difficult to play even on modern instruments?

As a fairly good one-time woodwind player, I marvel that clarinet players of, say, Rossini’s era could play his music on the clarinet of that day. Similarly, to play a lot of brass music on instruments that did not have valves astonishes me. Even today’s pianos are far more responsive that the old pianforte or early pianos.