Why do some docks only mirror displays on a MacBook instead of extending them?

This is the single most-cited frustration in the communities we read. Apple Silicon MacBooks (non-Pro/Max) natively drive only one external display over USB-C, so cheaper hubs that use a single HDMI stream can only mirror. Workarounds reviewers cite include true Thunderbolt docks (which can extend on Pro/Max chips) or DisplayLink-based docks that use a driver to add extra extended monitors, at the cost of some compression and CPU overhead.