Most banks don't have complex algorithms (the sort that some would consider OCaml or Haskell a good fit for) in general, they only have complex software because

Most banks don't have complex algorithms (the sort that some would consider OCaml or Haskell a good fit for) in general, they only have complex software because they have complex business and regulatory requirements, complex and incoherent existing systems to work with, cost constraints and organizational issues.