> It seems to me that all build systems are just DAGs

Yeah, well, it's a little bit more than that.

Two things come to mind that don't neatly fit the DAG mental framework:

    - dynamically generated dependencies (e.g. when you compile a C++ file only to discover that it #includes something and therefore has a dependency on that thing, and therefore the DAG has to be updated on the fly). Creating them by hand is horribly tedious, and/or borderline impossible (#includes that #include other #include ad infinitum)

    - reproducible builds, where a build system is capable of rebuilding a binary from scratch down to having not a single different bit in the final output assuming the leaves of the DAG haven't changed. A desirable feature that is darn near impossible to do unless you pair the DAG with something else.