I think that had to do with the Swan/Anderson collaborations being so good, that Giordano's inking talents were best utilized elsewhere.

But there must be at least one Swan/Giordano issue out there somewhere. George Perez when I spoke to him years ago let on that Curt Swan was a big influence on his work.
And that it was the thrill of his life to script a story and have Swan pencil a chapter (inked by McLeod) in WONDER WOMAN ANNUAL 1 during the Perez run. When he told me, I purchased a copy in the store, and asked Perez to sign it for me, which he was enthusiastic about.

There's also the Swan pencils/Perez inks in SUPERMAN 423, the first half of Moore's "Last Superman story".