Comment by jandrese

On opens source projects the build system needs to be reasonable enough that anybody can set it up. There are lots of conventions and even tools to help people. On closed source projects it is just Joe the sysadmin who sets up the machines for everybody working on it. Also, open source projects rarely include requirements like "buy a license of this specific version of this proprietary library and install it on your machine".

Doom had the advantage that it was written by a really excellent team with some standout programmers, and it has had plenty of people maintaining the codebase over the years.