Actually they do have a monopoly, That's the entire point of the tech cult and why they are so powerful and independent within the normally strict Imperial hierarchy. Without that monopoly there would be little reason for the Imperium at large to tolerate their shenanigans or bend to their whim as often as they do. It's also why military campaigns or mining operations can grind to a complete halt sometimes. Without a mechanicus agent around, no one knows how cogitators work or how to do serious repairs. Yes a guardsman can maintain their own lasrifle, sure. But the train would and could not run without a techpriest on board. And if things went wrong no one outside the cult would have any idea of how to fix the train if shit started to break. Your typical Imperial citizen may not even know how the plumbing in their hab-blocks works. They simply accept that it does work and trust that someone above them knows how to placate whatever machine spirit that keeps the water flowing. And the Death korp engineering corp is a military engineering corp. They aren't fixing generators and repairing tank engines so much as digging fortifcations and mining out trenchworks. Engineering corps in that context means they are experts at creating or breaking through a siege situation. Not repairing and maintaining equipment.