Two parts to this answer.
- ANTLR 4 does remove direct left recursion.
- In general, left-recursion elimination is more complicated for real grammars. ANTLR produces recursive-descent parsers which look very similar to the grammar they are generated from. Left-recursion elimination must consider predicates, embedded actions, arguments, local variables, and (for ANTLR 3) AST operations. The tool wasn't created because it was bound to be somewhere between incomplete (restrictions on the supported constructs), inaccurate (does not produce a parser that behaves properly), and unrealistically complicated.