Languages are constantly evolving -- and grammar is no exception. The way in which the brain processes language triggers adjustments. If the brain has to exert itself too much to cope with difficult case constructions, it usually simplifies them over time, as linguists demonstrate in a study on languages all over the world.
The grammar of languages keeps reorganizing itself. A prime example of this is the omission of case endings in the transition from Latin to Italian.Then how did language get complicated in the first place?