IMO this is mainly because of how the module system (or lack of) works.

No one wants to create and import 20 files for 20 DTOs, it creates a lot of indirection. I would use DTOs for everything if I could do:

// foo.php
final readonly class Options
{
    public function __construct(public string $foo) {}
}

final class SomethingWhatever
{
    public function doSomething(Options $options) {}
}

// bar.php
// somehow import the whole module, instead of just one class

$something = new Module.SomethingWhatever();
$something->doSomething(new Module.Options("no one wants to import 20 files for DTOs"));