For historical and cultural reasons; you could also ask why North and South America are considered separate continents and why the Arctic is considered a continent at all. The tradional seven-continents model predates modern geology and is a mix of geological, cultural, and linguistic categorization. We might also divide them by continental plates, in which case Eurasia and Africa are distininct continents. Another approach is contiguous landmasses, which leads to a four-continents model which has Afro-Eurasia, the Americas, Antarctica, and Australia along with their associated islands. There are various other models; which you prefer depends on your definition of continent and your reasons for dividing them.
Who considers the Arctic a continent? Wouldn't that be like considering the Pacific or Caribbean a continent? And many people consider North and South America to be the same continent: https://en.wikipedia.org/wiki/Continent#Number
