These idioms are of ancient origin, having been in use since maybe the 16th century, or even earlier. In that era, "a thousand" was considered a huge, almost infinite quantity (if someone had a thousand cattle, or a thousand dollars, he'd be considered fantastically rich) - people didn't throw around words like "a million" back then. Today, "a thousand," or even "a million," isn't big enough for idioms that stress quantity.
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