I have been in two software product houses for three years in a row.
The first is a small company maintaining a fairly small management system with a monolithic legacy code base (almost twenty years). Tightly coupled code is everywhere without sufficient unit test coverage. However, the management usually does not want developers to refactor the legacy code.
The second is a fairly big company maintaining a big domain-specific system with a huge monolithic Java legacy code base (over ten years). The layered architecture indeed decoupled the infrastructure from the business logic. However, in their business layer, there are also some giant classes with more than 3 thousand lines of code. Developers still continuously inject more and more code into those legacy classes. Developers are allowed to refactor their own fairly new code about adding new features, but are warned not to refactor these giant spaghetti classes, either. Experienced senior developers say that changes or refactoring on those classes might be disastrous due to the lack of regression tests.
However, personally I have read practical books about clean code and refactoring. Most of the books strongly recommend developers to refactor actively. But why in real world companies are against this?
So I would like to collect answers from very experienced developers. Why do these two companies I was in prefer to keep the super legacy code unrefactored? Isn't this disastrous?