I'm sure these managers exist, but there's plenty of places where they don't. My recent experience has been that the managers I work with, when asked, almost always say yes to extending technical work to do a better job and that it's inexperienced developers who either assume they can't have more time or don't know how to ask in an effective way; these are professional skills after all.
This doesn't mean that there's never a need to hurry, external deadlines exist, but these are the minority in a healthy organization.