Most wind and string instruments vary in volume (Some more than others) therefore without an expression pedal you can’t make them sound anything like how the real instruments play. (Many also change their sound s they get louder)

In addition when you are playing Melody and counter melody with your right and left hand simultaneously, (And bass if you have a set of pedals attached) you haven’t got any spare to move any physical sliders

Listen to any orchestra or band and you will find it is not a monotone sound but is a really expressive performance, something that can’t be done on any arranger without an expression pedal. (No matter what the sales blurb says)

Bill