Karatasi was borrowed from Arabic قَرْطَس (qarṭas, "paper, page; book; papyrus"), from Classical Syriac ܩܪܛܝܣܐ (qarṭīsā), from Ancient Greek χάρτης (khártēs, "sheet of paper; book"), of which also come English card, chart and charter, Spanish carta ("letter, missive"), French carte ("card; map") and charte ("charter"), Norwegian kart ("map"), German Karte ("card") and Russian карта (karta, "map; card"). The Romanian case is interesting, for it has two descendants of the Greek etymon: carte ("book", via Latin charta) and hârtie ("paper", through South Slavic chartija).
I think it's because karatasi is countable whereas paper is usually uncountable (unless using it to mean articles or documents). I think you use karatasi in plural to talk about lots of pieces of paper and in the singular to talk about one piece of paper or the material.
This should accept zao and yao though.