Because the right hand seldom knows what the left hand is doing. Policy at Beretta for many many years was to not release G models to the general public. That changed when Ernest got the Elite IIs built and sold, but then they stopped making Elites. Then they made some PX4s in G and released them, but Italy was unhappy with that. At some point people also discovered that Cougars could be converted simply by removing one spring loaded detent. Some more PX4s got released, and finally became a regular seller. Then the idea was floated to just make one slide style, and install whatever components a contract required, simpler, easier machining. But still no intention of releasing G model parts for these new slides to the general public. Meanwhile seeing all the conversions being done at various smiths and having entered into a build agreement with Wilson and Erney, someone got the bright idea to bring to market a kit they'd designed that would allow an easy changeover, but it had to get past the lawyers. All of these things occurred in compartmentalized units of Beretta. If I had a newer slide that would accept original G model parts, I'd be sourcing those instead of this new kit, if I had an old FS slide I'd be getting a new conversion for it. That's if they ever release the original style G model bits.
So yeah, a company that sells hundreds of millions of dollars of guns and such, is going to string you along for these parts....... seriously?