Tbf vegapunk had both Kaido's DNA and his DF DNA and still didn't created his seraphim version. For plot reasons oda only cloned OG Warlords. Otherwise Roger , Kaido, Garp , admirals etc were all would have seraphim versions.
It's the same about WB's DNA... it's heavily hinted that Weevil is a WB's clone in some form, so he probably tried it, for some reason failed (yeah, Weevil looks like a failure), then gave up to focus on other projects.
Btw, he didn't clone the OG Warlords... Kuma wasn't an OG, he replaced Hanafuda after Ace flattened him. So maybe it's just Oda wanting to have the original (creation-wise) warlords as Seraphims, as others have said.