Often the law as written is ambiguous and your implementation is based on interpretation of a specific regulatory body, or even a single auditor, which frequently changes. In a country as heterogenous as the US, you don't have "business logic" per se, but just a bundle of jurisdictional exceptions, that all change out of sync with each other an incompatible ways. You often need to keep multiple implementations of the same ruleset active in your system for different products, based on when they were initiated as well. Any "changes" are actually only additions of complexity, you almost never get to remove anything.

None of these things are strictly unique to finance but they are pervasive concerns there. Codebases sprawl for other reasons too, certainly. But they necessarily sprawl for this one in finance.