Why PHP's declare(strict_types=1); everywhere is impossible

app.php

<?php declare(strict_types=1);

require 'library.php';

public_library_function();

library.php

<?php

// Code you don't control and prefers coersive typing (e.g., not strict).
//
// public_library_function knows for a fact that $x is the string                                                                                      
// representation of a number. They would like to leverage coersive type                                                                               
// declarations in private_library_function to automatically "cast" to int. But                                                                        
// obviously *we* decide whether typing is coersive or strict.                                                                                         

function public_library_function() {
    $x = function_returning_a_string();
    private_library_function($x);
}

function private_library_function(int $x) {
}