In all that time, it is a fact that the number of times I HAVE PERSONALLY HEARD a unit from one jurisdiction speak directly to a unit of another jurisdiction is very low
This has more to do with political and operational aspects than technology. Back in the 1980s into the early 1990s most metro Atlanta (including CoA) utilized UHF conventional and rarely would one agency have programmed (or crystalized!) a radio for another except say, command staff.
Operationally, (more with law than fire discipline), working groups are closed and follow a command structure. Not defending the lack of daily cross-agency comms, but it can become chaotic if everyone just starts self-initiating and going "all over the dial" during an incident.
The key is a unified command. This doesn't exist in this area because agencies are separately run and don't have unified dispatch and command and control in most cases.
Where I work we have made great strides to change this. We have four cities that share a single talk group and dispatcher for dispatching of service calls. This means when an offense occurs in one jurisdiction, all agencies have situational awareness. They can also coordinate response.
As we are implementing a new CAD with switch to switch, this awareness will increase as we will have BOLO's and active calls for other jurisdictions who currently do not operate on our radio system.
These types of operational (and in many cases) political barriers, not so much technology, that take time and the right key people at the table to work out a "unity of command".
Those of us who focus on monitoring public safety communications often tend to have a different emphasis than those who use it for a living. I include myself in this category but we tend to focus on the LOGICAL use of communications ("why don't they instead.....").
And I will admit that, before the time I have put in at my agency first implementing our P-25 system and now managing the implementation of a new CAD and RMS system, it is easy to get caught on the side lines and armchair quarterback.
I will say that once one gets "inside", one gains an entirely new perspective and starts to understand why things are the way they are, and why changes are so slow to come. But rest assured, time and technology are marching forward and these changes will come.