Well, they don’t actually.
Poles conquered Muscowy in 1610, the only nation to have done so. And Russia or to be precise Muscowy at most ruled over half of the former Polish Lithuanian lands from 1831 to 1914.
Caste is different, you had the Hephthalites being added into Hindu society as Rajputs, etc.
You are so fond of the cast system that you must like the fact that for centuries Russians have rule over Poles and Ukrainians.