Why? It's standardized the project (thus facilitate use by others), manage dependencies, and provide a simple way to manage a project. Also, many other languages have already something that does that (gem, python package, jar) and not C.
2. It regularly breaks compatibility through regressions or feature removal, thus having to pin a specific version of autotools to a project to build it.
3. It doesn't manage dependencies.
4. It's not simple.
3. It does not download the dependencies but it can verify that your system has all it needs to build the project.
I don't have much experience with make and autotools so feel free to disagree with what I said and say why.