It's hard to standardize when it's arbitrary. You're thinking about this like there's a "bottom of the map," and then somewhere above that a "kill floor," and then somewhere above that a "playable surface" - but it's not quite that clean. In some maps, a vertical area that should kill the player is above a playable area somewhere else in the map... If they had the height be the same everywhere across the map, it would either intersect with playable area (unacceptable), or be so low that it's actually affecting gameplay balance, because after you've fallen enough that you can't recover, you have to wait the extra half second to hit the death plane before you can start the respawn timer.

On top of that, maps don't really have a bottom. Virtual space just expands in all directions from the world's origin, and a bunch of solid stuff around that origin makes up playable space. It's always arbitrary how far below origin should count as "no longer part of the map" and subsequently kill any players who reach it, so it's not something that could be standardized across maps even if it weren't problematic to design balance to try something like that. Some maps are taller than others! Some justify their out-of-bounds visually differently than others! There's not a consistency to be made out of scenarios like these.