If Buck built itself using Buck, then every time a change was made to Buck's source, the commit would have to include a new Buck binary that included that change. It would be easy to forget to include the binary, difficult to verify that it was the correct binary, and wasteful to bloat the Git history of the repository with binaries that could be rebuilt from source. Building Buck using Ant ensures we are always building from source, which is simpler to verify.
Also, because Ant is a more mature build system than Buck, it has support for features that we have not had time to include in Buck yet, such as generating Javadoc, static analysis via PMD, Python unit tests, etc.
That said, as a sanity check, Buck is capable of building itself. Once you build Buck using Ant, you can re-build Buck using Buck by running
./bin/buck build buck.