In a lot of cases the source code did not really exist in a meaningful way - lots of people on 8-bits wrote source code in a machine code monitor, with no comments or labels of any kind - rather than in a macro-assembler.
And even when they did, the source often stayed with the author rather than with the distributor, and may only have been "archived" on a single floppy in their personal collections, which may have stopped working, ended up in an attic or gotten thrown away.
Keep in mind that many of the early games were "throwaway" projects of a few weeks or months of freelance work for the people involved, and often conversions from other platforms where the lack of importance ascribed to source was demonstrated in that many conversions were done without access to source code or even design documents. E.g. some arcade conversions were done by playing the arcade game a lot to figure out the graphics and level designs and overall musical style....
For a lot of these games, your best bet is to crack out a disassembler and reverse engineer them. On the upside, most of the ancient games are tiny and fairly easy to figure out. E.g. for C64 games, use a freezer cartridge, and check the address stored in $EA31 and $EA32. It contains the address of the raster-interrupt. Almost everything in most C64 games hangs off the raster interrupt. That + 6502 assembly + a book explaining the C64 registers is all you need to get a good start at reverse engineering the workings of 90%+ of C64 games, and most of them will consists of at most a few thousand lines of code (The rest includes various oddities - e.g. Sid Meier's Pirates for C64 consisted of high level logic written in BASIC plus a bunch of asm subroutines to speed things up and/or save space).
Well ID does not really do it, you should rather say Carmack. Now he's gone and we don't know if ID will continue to release source code in the future. Besides, I was referring to older games, in my book anything from Doom and onwards is relatively "recent" in the history of videogames.
