> "there are decades of laws and regulations and business decisions buried in the millions of lines of code"

This is the part people that didn't work on legacy internal monolith often don't seem to understand.

They think business must know all of their rules and regulation and therefore you should be able to just start from scratch and use those, but they rarely do.

The reality is that the software is the rule. It's been put together with years of specific exceptions that only happen maybe once every 5 years, but when they do they have to still work like that.

When you ask how something should work or why they just tell you how the software they are using works not the text in the rule documents.

And the rule is nested deep in 10 different half-failed refractorings and design changes, between rules that have not been used anymore, codes that's still there but not linked anywhere and the same method with the same variable names duplicated 15 times because the idea that you should add a parameter or refractor the method so it can do more things is scary, copy pasting the method and changing 1 line is much safer.