Even though it is "probable" even above the 99% level, he is still not officially nominated by the Republican convention. Even after he has 1,237 delegates committed to vote for him (no matter which ballot), that would only make him the presumptive nominee. That is, we assume that he will become the nominee, but he has not yet become so.
Google "Republican presumptive nominee" and you will see the term used.
After all, a meteorite could still (G0d forbid) hit the airplane Mr. Trump is in before the convention.