Granted, there are ways to interconnect products made by different manufacturers without a standard protocol but it is not easy.
A single front end system can be made which "knows" the proprietary communications technique of each system, effectively bridging their differences in software. This approach requires the cooperation of multiple manufacturers, or reverse engineering. In practice these systems have been very costly to implement and support, as the various systems evolve and change independently.
Another method involves defining a common protocol between a centralized front end and individual "gateways" to proprietary systems. This approach has also proven to be costly, and was not really supported by vendors, many of whom were threatened by "open systems" thinking.
The end result is that building owners have often felt trapped, locked-in to products of a single manufacturer.