why isn't it קסם instead?
Magic is קסם.
However, in Israel that word is also used in plural - "קסמים". This translates into "magics", which is not used in English. Sometimes it happens that a single form in one language translates into a plural form in another.
Magics is used in English, just not as often.
Think of it a s "magic trick" vs "magic tricks".
Kind of like "maths" which seems to be the norm outside the U.S.
This word seems to be a cognate to the Arabic قسم meaning oath. Is there any relation?
Yes, it is the same root as e.g. in kismet, foreordained fate. The original meaning of the root QSM was cut, break, distribute, especially divide by drawing lots at the sanctuary.