Most old lead objects are cast, casting is a process where a liquid is poured into a mold where it is left to solidify. Presumably because both the melting crucible and the cast are under the influence of gravity while the metal is in the liquid phase the heavier elements will sort themselves to one end of the end-product, maybe even into the flues that will get cut off from the final product.

It would be much harder to keep the U238 evenly distributed throughout the lead during such a process.