DustyStyx: Because Atari is just a publisher, not a developer. They would have to hire some one that can read what code is what to make Blood work with the open source version of Build, sans Bink* codecs.
Honestly the only way I think we would see Blood source code would be if it were "leaked".
EDIT: Bink, not Blink.
http://www.radgametools.com/bnkmain.htm They wouldn't have to do anything of the kind. What they would have to do is 1. Consider if the IP is valuable enough that releasing source code wouldn't undermine business motivations and 2. Have legal team draft a license and
possibly 3. Remove any proprietary under-license software development libraries they use (such as Bink, etc).
The problem is releasing source code is not a cut-and-dry thing. It treks through much legal territory, all of which costs money, and the motivation and payoff often doesn't warrant the effort for them.
I asked John Carmack if he'd release the source code to Keen not too long ago and he said he would but he wouldn't push Zenimax to do it at this point. Basically, Zenimax would need to pay to do it and there's little upside for them. John Romero said he would and that he thought he had the source too, but it would need id's blessing, which doesn't seem forthcoming.
This is the problem when something has gone too long without having had the source released. It becomes a legal jungle to be navigated in order to put it out, and most of these companies are more concerned with what can make money immediately than a fan community.