>>528026436
The Arbites has a strict and robust structure, as well as resources properly allocated throughout the Imperium. Though the Inquisition has high authority, inquisitors are a bunch of paranoid haphazard yahoos whose resources are mostly limited by their personal influence and what's available from local requisition. Some militant Inquisitors may maintain a large force, but most don't to make investigation easier and simply commandeer as needed. Grendyl seems keen on his information network of agents, and simply throwing his badge around when he needs more.
Also the High Marshal of the Arbites has a seat on the High Lords of Terra, making both organizations represented at that level.