Because before XIX century are Ruthenians/Ruses/Russians had common although diverse identity. Separatist Ukrainian identity is a XIX c invention rooted in Austria ruled Lvov area. BTW, most of Ukrainian Christians are in the "Muscovy church" as well.
And why is Lemko most similar to Ukrainian?
Because they are closely related.
"Separatist Ukrainian identity is a XIX c invention "
Ever hear of the Zaphorosti Cossacks of 3 centuries prior?
Urkainian identity as a Autrian invention is laughable - did they clone Shevchenko?