Corals are killed en masse by other events, particularly cyclones. These have a much bigger affect on coral coverage, in the short term, than ocean temperatures / bleaching (which is transient and more localized).

Bleaching is not death of the coral, it is the expulsion of algae (typically in high temperatures). The coral can find a different species of algae, or the algae can return once temperatures drop.

There are corals living in regions of the ocean with different temperatures, so it is not that high temperature alone would kill all coral, but populations adapted to a given region (like north vs south GBR) are being affected by the speed of the changes.