[njonsson]

Simpler, yes, in the sense that just one instrument (the soloist) would have to play notes they were not prepared to play. But a concerto (typically a long piece for one or more solo instruments accompanied by orchestra) showcases the solo instrument, both by exposing it and by demanding much from the soloist’s technique and interpretive powers. It’s a shorter order for the orchestra to adapt than for the soloist to do so.