It's harder to ensure that the grammar doesn't have unintended ambiguities when using a parser generator. That doesn't matter as much when "just" developing another implementation of an existing fully specced language, but for cases like the examples you cite that doesn't exist.

That issue is the primary reason why postgres continues to use bison. The SQL standard introduced potential parsing ambiguities frequently and leaves a large portion of the things necessary for a functioning RDBMS unspecced.