Here's why:

* Novae are temporary events: A nova is a sudden brightening of a star, caused by a thermonuclear explosion on the surface of a white dwarf star. This brightening is temporary; it fades over time.

* They are not the brightest stars in the universe: While novae can become extremely bright, they are not as bright as other astronomical objects like supernovae, quasars, or the most luminous stars.

The brightest stars in the universe are:

* Supergiants: These are stars in the final stages of their lives that are vastly larger and more luminous than our sun.

* Hypergiants: Even larger and brighter than supergiants, these stars are extremely rare and short-lived.

* Quasars: These are supermassive black holes at the center of galaxies that are powered by accretion disks. They are the brightest known objects in the universe.

So, while a nova can appear incredibly bright, it is not the brightest star in the universe.