>>2091639'cause it's company property so devs don't hang onto it after the project for legal reasons.
At that point it become management's job, except they don't know the difference between the source code and any other random document so it just get pushed somewhere in a pile with everything else - then since no manager give a shit about dead projects the "somewhere" get quickly forgotten.
One hard-drive wipe (to "save cost and re-use company hardware") or location change later, and the thing will effectively be nowhere to be found.
Archiving isn't a trivial task that get done automagically.