Why do cowboys call inexperienced riders "greenhorns"?

"Green" is a general term for anyone (human or animal) that's inexperienced, but why is an inexperienced hand a greenhorn?

Simple.

If you aren't very good at riding and are in a western saddle, the temptation is to grab the horn. A real cowhand would never do that - they don't need to. So it shows you up as a novice (except in those serious "oh shit" moments when you just hope nobody's actually watching).