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Why can a quasar outshine the stars in its host galaxy?

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six quasar host galaxies — Quasar Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).

A quasar can outshine the stars in its host galaxy because it is an extreme example of an active galactic nucleus powered by the gravitational accretion of matter onto a supermassive black hole.412

As matter—such as gas or stars—falls toward the central black hole, it forms a rapidly rotating accretion disk. Due to turbulence and viscosity, this material loses energy and angular momentum, causing it to spiral inward. This process generates immense heat, with temperatures reaching millions of kelvins, resulting in the emission of prodigious amounts of radiation across the electromagnetic spectrum.321

While a quasar is extremely compact—roughly the size of the solar system—this efficient conversion of gravitational energy into radiation allows it to emit up to 100 times as much light as an entire galaxy containing billions of stars.425