php mysql syntax with IN

Question

Why dosnt this work:

$mystr = "1,2";
$stmt = $pdo->prepare('SELECT * FROM mytable WHERE mycol IN (:mystr)');
$stmt->execute(['mystr' => $mystr]);`

When this does:

$mystr = "1,2";
$stmt = $pdo->prepare('SELECT * FROM mytable WHERE mycol IN ('.$mystr.')');
$stmt->execute();

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