Because the Hive and Vex are transcendent beings themselves. Oryx gained the ability to create a disease that perverts and distorts reality itself. The Precursors are the only ones who could match the Hive Pantheon's reality alteration. The UNSC, Covenant, and Forerunners might be able to hold out for a while, but in the end they're fighting the Hive, their own men (but even more powerful,) and a virtually infinite army of Vex. It's not so much a question of "could they beat the Hive and Vex" so much as it is "could the UNSC and Covenant fight empowered Prometheans?"