Or biases inherited from deploying on single or dual core 32-bit systems from 20 years ago.

Honestly, it's a mostly obsolete approach. OS threads are fast. We have lots of cores. The cost of bouncing around on the same core and losing L1 cache coherence is higher than the cost of firing up a new OS thread that could land on a new core.

The kernel scheduler gets tuned. Language specific async runtimes are unlikely to see so many eyeballs.

Also very useful for applications that have heavily animated graphical user interfaces, where you don't want I/O to interfere with UI animations on the UI thread. (e.g. ALL android apps. Or any UI framework that has a message pump (all of them?).