Same reason we had to have a Miranda decision, despite an extant 5th Amendment....or Heller, or laws explicitly permitting open carry, in spite of the presence of the 2nd...
Because agents of the government will find ways to violate our rights until and unless they are explicitly and enforceably forbidden to do so.
Becasue that's what they do.
More likely, lawyers elected to a legislature can not keep their self interest at bay and craft laws that enrich lawyers. While judges, who are also lawyers, mold precedents to the same purpose just as fortold in the “Anti-Federalist Papers”