Because all of United’s debt is commercial, much of it via the initial leveraged purchase, but more still from subsequent loan restructuring and cash injections. The Glazers have not used shareholder loans to finance club operations, as that comes with a set of risks they most certainly would not wish to assume. I believe they have effectively made equity investments their public listing in the US, which as supported further leveraging of operations.
This isn’t a sign of good management, though. They’ve saddled United with significant debt servicing obligations and continually extract value from the club whenever possible.