Short explanation: A person must already be a saint before being canonized.
More complete explanation:
Canonization is nothing more than the Church’s guarantee that a person already is in heaven; it does not make them a saint. Otherwise, canonization would yank a person out of Purgatory or Hell and throw them into Heaven, but this is impossible, since (a) souls in Purgatory cannot work miracles, but canonization requires that a miracle occur through the intercession of the saint; and (b) souls in Hell cannot leave. Therefore, a person who is canonized is already in Heaven. Canonization is nothing more than the Church’s infallible guarantee of that fact.
Related Catholic Trivia: There is no “opposite” of canonization. That is, the Church has never proclaimed that any particular human being is in Hell. Also, since canonization is infallible, the Church cannot “un-canonize” or “de-saint” anybody. For example, nothing concrete is known about Saint Christopher, so the Church removed him from the official calendar of feast days, and made Saint Raphael the new “patron saint” of travellers... but this in no way “un-canonized” Saint Christopher, even though the mass media reported it that way, and many Catholics have that misconception. So don’t throw away your Saint Christopher medal; he’s still a saint, and is still a patron saint of travellers (along with Saint Raphael!).