Why is the antonym of "within" not "without"
Solution 1:
As an antonym of within, without is an archaic, literary form:
outside (prep.):
- "the barbarians without the gates"
outside (adverb):
- "the enemy without"
Without:
Old English wiðutan "outside of, from outside," literally "against the outside" (opposite of within), see with + out (adv.).
As a word expressing lack or want of something (opposite of with), attested from c. 1200. In use by late 14c. as a conjunction, short for without that.
(Etymonline)