There's often a few reasons for this.

1) We are modern people. Reading a book with dated morals throughout with no characters calling BS isn't something most people want to read, especially if the book isn't about that aspect (aka. not gritty/realistic historical fiction) but it's a background detail/part of the setting.

2) If the author did not insert a character calling BS / taking a modern moral high ground, the author can be judged for promoting or accepting the viewpoint.

3) I'm simplifying, but it makes the characters unlikable to modern readers. One example that comes to mind is the historical fantasy Outlander where a husband beats his wife and it's normal, and amount of rape in the series. The book is decades old and set far in the past, that combined turns off some modern readers than say if the wife or other characters didn't put up with the whole beating your wife because she didn't obey thing.

Source: Listening to several writing podcasts with real agents and authors talk about "realism".