For one AVRs are really cheap and neat, and a few cents can add up, e.g., if you plan to sell 10k+ devices. Sometimes it's also just what you already have lying around, so better use that then to let it get covered with dust.

Some anecdotes for when I assisted on a µC course at my university where we used AVRs:

We also had one practical assigned with FFT to read out the RPM of a fan via a microphone input sampled by an ADC to detect blocking of the fan (as fallback for early detection of cooling failures in a simulated processing plant), with some user interface displayed on a GLCD on top of it; was quite a pleasure to see the tiny thing handle all that in realtime and still provide a fluid user interface experience.

Another assignment was creating a lunar lander like game controlled by Wii/Nintendo nun chucks with sound (read from SDCard over SPI and then routed to an external sound module over SPI, DMA would have been great so to say), and high scores send over an external SPI attached Ethernet module.

The game required fixed point arithmetic too for its physics, and it truly learned students that parallelism and concurrency are two different things and that one simple core can already do a lot of cool stuff.

Same reason stories are about running Doom on underpowered devices and not on GPUs. For the challenge.

Running FFT on powerful devices with FP would not be a story.

For the same reason as I implemented a VA synth on an Arduino with an antialiased oscillator and 2-pole resonant filter - because building a Ship In A Bottle is fun!