Why is pure literal elimination absent in DPLL-based algorithms like Chaff?
Solvers that use the two-watched-literals algorithm to implement unit propagation don't keep track of which clauses have been deleted (by implication) to produce the subformula implied by the current partial assignment.  By not tracking this information, solvers can avoid touching most of the clauses during assignments and avoid touching any of the clauses during backtracking.  Figuring out which variables are currently pure in the formula means losing these substantial efficiency gains.  Meanwhile there's nothing to suggest that pure literal removal will produce conflicts or satisfying assignments early enough to make up for the heavy tracking cost.

Solvers that do formula simplification before starting the search procedure will generally include pure literal removal since it is quite cheap to do as a preprocessing step and could potentially remove many clauses.