It makes it so much easier for attackers - they then can know that a user has an account there too, and can then use that information to find that user's other known passwords. It's offering up information the attacker might not have.
Wordpress takes almost the opposite approach: They consider the fact that you are an author on a page public information. There is even the /wp-json/wp/v2/users api to enumerate them At least they don't show email addresses, but you can also login with the username. ([1],[2] for examples in the wild).
Of course as a wordpress admin you can decide differently: extensibility is one of the core pillars of wordpress. Security arguably is placed behind extensibility and user-friendliness.
However, I stand that it does depend on the application. For example: Facebook does not use generic error messages. I presume because there are other trivial ways to find out if a user has an account so mitigating enumeration through the login form is not actually adding extra security.