If they're integrated into the OS they have to be supported at the same level as the rest of the OS, which entails the same rigorous processes w.r.t. updates, bug fixes, etc...

By keeping them out-of-band, that support requirement doesn't apply, allowing the tools to be much more aggressively updated and released without the same degree of oversight. There's a reason they're licensed separately and effectively with no support or warranty. Doing so enables their rapid development without/less-of the usual bureaucracy.