It also has to do with monster AI. When monsters use basic attack they mostly just stand still. Environmental attacks is an easy way to create fights which are based on patterns. And in a fight which is based on patterns, there need to be easy parts where players can relax.
In Diablo monsters have always been fairly simple, but I think it would be nice to see some monsters which react to player actions. For example there could be a small chance that a boss does something different when a debuff is cast or when someone uses a defensive skill etc.