Most in-house game engines built after a certain point use a non-trivial amount of third party code, including console stuff which is under strict NDA, so it's a huge hassle to open source them. Most iterations of the Source engine use Havok physics for example.
IdTech probably was only open sourced because Carmack pushed for it, but it helps that IdTech of that vintage was all in-house code exclusively targeting the PC. I think the only thing they had to cut out for legal reasons was the patented shadowing algorithm in Doom 3.
IdTech probably was only open sourced because Carmack pushed for it, but it helps that IdTech of that vintage was all in-house code exclusively targeting the PC. I think the only thing they had to cut out for legal reasons was the patented shadowing algorithm in Doom 3.