ID Software released (at one time) the source to older titles. Why can't GOG do/push for that too?
Another reason is that third-party libraries often aren't free software. For example, the big three console makers are known for banning copylefted software on their platforms. The original source release of Doom was silent because Id Software had licensed a non-free audio library from a third party. (Source ports ended up replacing it with a shim around Allegro or SDL.) Id had to rewrite the Doom 3 engine to eliminate a patented "depth fail" shadow volume processing technique invented by William Bilodeau and Michael Songy of Creative Labs before its source could be released.
I'm not about to compromise my machine my running proprietary software on it.
Then how does it connect to the Internet? All cellular radios and many WLAN radios contain a microcontroller running non-free software. And how does it boot? Most commodity PCs ship with a proprietary implementation of EFI and not coreboot.