there are people who have. Feel free to look them up.

This would be more impressive if it didn't so often happen that the ones with the best-sounding evidence so often turn out to be outright fraudulent. E.g., Eben Alexander's book ("Proof of Heaven") makes claims about his illness that are demonstrably untrue, and it turns out he's been in trouble before for reasons that call his integrity seriously into question (e.g., there is reason to think he's falsified patients' medical records); Alex Malarkey ("The Boy who went to Heaven") retracted his claims to have died and visited heaven.

one of the requirements of canonisation

Yeah, they do indeed require evidence sufficient to convince the church that the person is in the afterlife. Unfortunately, it doesn't seem like the church needs terribly good evidence. Typically what they want to see is a miracle performed by the proto-saint's intercession. E.g., the miracle that qualified the former Pope John Paul II for beatification (he hasn't been canonized yet) is that a nun had a neurological condition, she prayed for him to intercede for her, and she stopped having symptoms. But (1) no one actually knows exactly what condition she has or had, and hence no one knows how likely remission is even without divine intervention, and (2) she appears to have had a relapse since the alleged miracle.