Why isn't there a straightforward answer to the question, "are you legally eligible to work in Canada?"

I keep asking the internet search engines and they keep talking about birth certificates and visas. I just started looking in to this today. What qualifies as "legally eligible"? I'm the most squeaky clean and boring American citizen around. What could disqualify me? Is being squeaky clean enough to be legally eligible? Or do I need to have a visa in hand?

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