"But why should it be a detail we have to read for carefully & look for clues?"
It... isn't? I mean, you aren't obligated to read anything carefully if you don't want to. (Though obviously that's going to significantly impact your understanding of whatever it is you're reading.) But if you're asking why it's not stated directly in plain language, i.e. "Shadow is a man of mixed race", then... I don't really understand. Stories often contain details that aren't explicit, that's part of the point in reading them and figuring out what's going on. You might as well ask why Gaiman doesn't come right out and say that Low-Key is the god Loki instead of letting people make that connection for themselves. Or why Gaiman waits to make that Mr. Wednesday = Odin connection. Or why Gaiman doesn't put in a footnote that says, "Mr. Nancy is actually Anansi, a West African trickster god who often takes the form of a spider." Why doesn't he simply tell us that Hinzelmann is the reason why the children of Lakeside are vanishing and that Hinzelmann too is a mythological figure instead of the kindly, helpful man he appears to be on the surface? Why not state upfront what all the mythological identities of all the characters are?
So we circle back to your question.. what does he accomplish by having the readers "read carefully & look for clues?" That's just how books and stories work: some details are plain, some aren't. The discovery and grasping the significance through your own efforts is part of the fun, surely.
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It... isn't? I mean, you aren't obligated to read anything carefully if you don't want to. (Though obviously that's going to significantly impact your understanding of whatever it is you're reading.) But if you're asking why it's not stated directly in plain language, i.e. "Shadow is a man of mixed race", then... I don't really understand. Stories often contain details that aren't explicit, that's part of the point in reading them and figuring out what's going on. You might as well ask why Gaiman doesn't come right out and say that Low-Key is the god Loki instead of letting people make that connection for themselves. Or why Gaiman waits to make that Mr. Wednesday = Odin connection. Or why Gaiman doesn't put in a footnote that says, "Mr. Nancy is actually Anansi, a West African trickster god who often takes the form of a spider." Why doesn't he simply tell us that Hinzelmann is the reason why the children of Lakeside are vanishing and that Hinzelmann too is a mythological figure instead of the kindly, helpful man he appears to be on the surface? Why not state upfront what all the mythological identities of all the characters are?
So we circle back to your question.. what does he accomplish by having the readers "read carefully & look for clues?" That's just how books and stories work: some details are plain, some aren't. The discovery and grasping the significance through your own efforts is part of the fun, surely.