Why do we use excess reactants in a chemical reaction?

Why are percent yields usually less than 100%?

Is it because we want to maximize the amount of products produced, and reactions usually end up in a loss of reactants to the air, etc?

2 answers

generally, excess reactants are used to drive the speed of reaction,or the equilibrium toward the desired product.

Yield is usually less than 100 percent because of temperature is not ideal, pressure is not ideal, reaction medium was a trade off, and a mydrid of other factors.
Everything Bob Pursley said is ttue. It boils down to two ways of saying it.
1. There is no free lunch.
2. Nothing is 100% anything.