Optimizability is one of the core ideas of the relational model. The others being data independence and a declarative query language. SQL is based on relational calculus and relational algebra, which in turn are based on first-order logic. The reason that every attribute in the relational model must be atomic is that first-order logic cannot be applied to nested sets. However, first-order logic is what makes query optimization possible in the first place. Thus, if one were to develop a query language that did not have these constraints, then one would lose optimizability.