I'll add my two cents here. A lot of the work that goes into a codebase doesn't necessarily appear in the final product. Sometimes code is hastily written to get the system working, and then it has to be rewritten to be faster or more stable or what have you. A rewrite can just reimplement the final code. Sometimes the company changes direction (think of the stereotypical aimless startup that pivots every Tuesday) and the code solves a problem that the company is no longer interested in. A rewrite can just ignore that code. Some of this has to do with technical debt, but it's also due to the fact that it's easier to do something when you know exactly what you're trying to do.

It's like the difference between between reproducing a proof from a math textbook and actually proving a new theorem yourself. The latter takes a lot more time and effort, even though the amount of work in both cases looks the same from the outside.