Right, but that's part of the point. Apple should just have this inside macOS.

From MonitorControl's GitHub README, emphasis mine:

> Most modern LCD displays from all major manufacturers supported implemented DDC/CI protocol via USB-C, DisplayPort, HDMI, DVI or VGA to allow for hardware backlight and volume control.

And a blog entry by the author of Lunar, an app mentioned elsewhere for this problem: https://alinpanaitiu.com/blog/journey-to-ddc-on-m1-macs/

These are not examples of "just works". Apple intentionally does these things to cripple integration into non-Apple products.