while calling Persians leader after Khosrow never really became a trend, even in Persia Itself, Emporaros calling themselves a variation of the Title Caesar become popular. albeit way, Way later in history. why Did the Arabs do it... and that early!
Is it for example that Arabic simply did not have a word for Shah or Emperor or maybe the Quran came up with/popularized those title?
also, Arabs were loyal Allies to Persia and a Major trade partner to the Romans, so it's like they were foreigners who had no Idea what they were called!?
Caesar was a title long before the Arabs used it. When Trajan succeeded Nerva to the emperorship, he used the title Imperator Caesar Nerva Traianus Divi Nervae filius Augustus.