Why is this property of the Cron service initialized in this manner?
It's backward compatibility.

Strictly speaking, constructors are excluded from BC, but the reality is that it is fairly common that one has to override them to inject additional dependencies.

The fallback to the global service means that if such a child class calls the constructor without that argument, it is not going to fatal.

That's especially common for base classes like ContentEntityForm, ControllerBase and other base classes that are specifically designed to be subclassed.

It can also be helpful during updating from an old version, because when running update.php in the UI, the class might be instantiated before Drupal has a chance to rebuild the container and provide the new arguments.