A Total Party Kill is hard for GMs, especially ones that invest a lot of time and work into the campaign. The villain has an evil scheme that needs to be thwarted, which might tie into the backstory and motivations of the party. There are NPCs with relationships to the PCs. There are personal quests and all the little subplots.

All that hard works goes to waste if the party does something stupid and all die.

When a GM has to choose between their grand story and its rich tapestry of plots and characters and the results of a random dice roll, it’s really tempting to change things in favour of the players. Oh, that crit is just a normal hit, or their AC just dropped by 5 points because a buff spell expired. The adventure must go on!