Gimbal motors operate best for a known weight, the gimbal software drives the motors based on that weight, and balance is critical for proper function. Add in camera-dependent components (e.g., mounting points, hdmi plug type, power input type, lanc controller or LED controller, etc.). So, products like the Zenmuse specify a specific camera and lens. There are plenty of options for adjustable gimbals, but by their nature they target/require a user more into tinkering with settings, balancing the system correctly, etc.