Because to me, I thought the point of the very first conversation Rick and Shane have before Rick gets shot was to telegraph that Shane is actually more what the wife is looking for anyway.
I thought so, too, especially after rewatching it this past Sunday. Also, Shane was definitely interested in the wife before the epidemic. He might not have done anything about it, but he was interested. He tenses up noticeably when Rick is talking about what Lori said.