It may have been due to multiple manufacturer parts & their stack-up tolerances for an adequate fit.
..... or possibly to reduce the number of sub-assemblies on the production lines to accommodate all possible combinations that were built as sub-assemblies.
..... or a marketing call, no-one wants a chrome grill in a car they just ordered with de-chromed bumpers.

Why aren't there black, gold, etc. "rubber bumpers"? Same answer!