Some folks would have to spend so much time ripping things out of Ubuntu or Fedora etc that it's much easier to build Arch with only what they want. There are other benefits, like everything else not everyone cares about the same things.
If you feel like no distro does things the way you'd prefer, Arch may be for you. If you have no complaints about whatever distro you use, there's probably not any reason to jump ship to Arch.