>>723700792
Because it isn't based on the original engine anymore.
GZDoom is what's known as a "sourceport." In essence, it's using the original source code to make what's effectively a new engine (or new iteration of the original) that's able to run the intended game, but with new capabilities for how it can be played or for how people can make content for it.
In the case of DOOM, this can be things like being easier to run on modern systems and modern resolutions, or to make the multiplayer be more akin to a modern shooter. In the case of GZDoom, one of the biggest features is ZScript, a programming language that basically lets you do whatever you want while still having compatibility with DOOM, creating a powerful foundation where you can mix and match content from the past 30 years.
It's stuff like that as to why it keeps getting updated. After a certain point, it's basically an entirely new thing that's able to run DOOM, rather than a DOOM engine, but it still goes a bit beyond that compared to, say, being able to run DOOM in Garry's Mod or Majora's Mask
