Which is a pity because "Large Steppe Empire" is a repeating pattern that happened constantly in the Steppe since before Christ
Coding a generic game mechanic to provide for a specific scenario is too costly, even if it's a scenario that repeats once in a few centuries. Generic mechanics that cover recurring scenarios that repeated multiple times over a (relatively) narrow timespan, for different global players: colonization, industrialization, power blocks.
It's not a mere "specific scenario". The dynamics of Eurasia from before Christ to until Russia broke the Tartar yoke, were strongly dictated and changed by the rise and fall of the Steppe Dwellers. Tribal confederacies snowballing into large hegemonies of Steppe Nomads and Semi-Nomadic Conquerors who then start rolling over nearby civilizations, is something that has been happening for thousands of years and has strongly shaped the development of the World-Island. Scythians, Cimmerians, Sarmatians, Xiongnu, Gokturks, Huns, Hephthalites, Rouran, Kara-Khitai/Liao, Avars, Magyars, Bulgars, Seljuks, Ghaznavids, Mongols, Timurids, Dzungars... it's a constant pattern that keeps happening over and over, and only stopped when large gunpowder armies became a thing among the settled civilizations.
This has strongly shaped the rise, development and fall of almost all the great civilizations in Eurasia. The shadow of the Steppe Dwellers, for long millenia, loomed large over the settled civilizations of Eurasia.
Essentially, if your game spans somewhere from the time of Alexander to the 1600s, your game needs to be able to portray the significance of the Steppe Dwellers. Angry horse-riding dudes raiding and invading from the Eurasian steppe to your territory is a big deal.