Actually it is the converse of I 17, and it seemed more like a proposition than a postulate .
Moreover, Euclid made no use of the parallel postulate until he reached Proposition I 29. It was natural to wonder if the postulate was really needed at all, and to think that perhaps it could be derived as a theorem from the remaining nine "axioms" and 'postulate", or, at late, that it could be replaced by a more acceptable equivalent.