People who write a language simply don't like bringing in another one, and that's all it is.

If we have a compiler for language X in language X, and don't use any tools such as a parser or lexer generator, then there is nothing but code in X in the project, and that makes the developer of language X feel like they have earned major brownie points ... err, I mean, ... that they have kept their project free of cumbersome dependencies.

If language X is a one-implementation invention, then the tooling won't exist which hits these checkboxes: (1) is written in X; (2) generates code for X. By the time language X is mature enough that its ecosystem has something like that, it is long past the point where it would make sense to introduce it into its one and only implementation. There would have to be interest in writing another implementation.

E.g. the first C compilers would never have used Yacc.

Among languages that have one implementation, and that use parser generators, we will almost always see that another language is used for bootstrapping and the generator is for that language.

For some designers, that is a bruise to the ego, or else an unappetizing dependency. Even if they are boostrapping with another language, they are thinking forward to a future release where they will ditch that: they will rewrite parts that are in the boostrapping language in the new language to make it self-hosting.

If you use tooling like parser generation, which is in the ecosystem of, and oriented toward, the to-be-jettisoned-one-day boostrapping language, that throws a barrier in the path toward self-hosting. So you tend not to do it.

Like if you are boostrapping with C, and have it in the back of your mind to get rid of it, do you want to be bringing in a complicated tool with its own input language, which generates C? You think twice and are more likely to go ahead if you've resigned yourself to sticking with the C dependency.