An issue, though not an unsolvable one, is that you probably need a standardized representation of the AST to write these optimization passes against; it is not

An issue, though not an unsolvable one, is that you probably need a standardized representation of the AST to write these optimization passes against; it is not uncommon for compiler writers to disagree on what representation is the best for what purpose. Then there is the question of optimizations that are (typically) easier/possible only at the code-generation phase. All of this is solvable of course, but there needs to be some will to do so (or in the case of companies, enough commercial benefit).