The only case of mkv to dat conversion that actually makes sense is export of Matroska videos (MKV) to MPEG video format represented as .dat file. This is probably done for legacy reasons, typically in order to playback modern video on old device or machine.
Many video editors and converters should be able to help you with this, especially since both formats are well-supported. Depending to the settings and adjustments, the resulting video can be either of greater file size or of reduced quality.
Updated: October 26, 2023