If you have two audio streams, you can't play them as is on the audio device, you have to mix them together. The same happens with analog speakers as you can't just add two signals together. I believe at one point with Alsa, when an application takes control of the audio device, no one else could play with it. Now Alsa comes with dmix (a digital mixer feature) enabled in its default configuration, so two applications may play how they want. And we have PulseAudio, Jack, and Pipewire on top of Alsa to add more features.

OpenBSD still present raw audio devices, but they have sndio which provides a more helpful interface for applications including resampling (not the best algorithms there, according to them).