Every modern keyboard uses a general purpose MCU anyway (formerly AVR, now usually ARM) so it's not like they're adding any complexity by using MCUs, and regard

Every modern keyboard uses a general purpose MCU anyway (formerly AVR, now usually ARM) so it's not like they're adding any complexity by using MCUs, and regardless of how the key scanning is implemented you would still want a programmable processor to support all of the layers and macros required to make such a small keyboard practical. Some early iterations of split keyboards did use a parallel bus between the halves instead, which allowed them to have a single matrix spanning both sides which was read by a single MCU, but that went out of style since the cables were unwieldy and MCUs are cheap. Now they all use two MCUs linked over serial, or wireless.