Lets forget about Ant here. There are tons of better tools out there. Lets assume you are talking about maven. Build tools are not tooling libraries. They have one and only one job, to generate a final deployable "unit" (war/jar in case of Java, binaries in case of C++/Go etc.). They don't decide how you create your tooling libraries, hell they use the same libraries that probably you will use to create these "units". So what is the advantage then?
Separation of concern: The idea here is, the development aspect should not be part of your build system. eg. Maven lets you create artefacts without worrying about how it is going to be. It is maven's responsibility, not yours.
Dependency Management: Possibly the most important feature here is this one. There are situations that you will face when you have 2 different incompatible versions of library, how would you manage it. One simple solution is to keep the libraries with you, but then what if you changed your machine or someone else has to use it, will you ask them to download the libraries again
Shipping: This stems out of the previous point. To avoid keeping libraries with you and shipping them as part of source code, there are repositories which contain the versions of these libraries and these build systems are responsible for pulling the correct version of library based on a config. Now all you need to do is ship your code with a config and it is ready to build.
Now, tell me, do you really think for a project with huge number of files and loads of dependencies, is it really easier to add yet another aspect of development, when you have so many other things to worry about?