The Cabal factions that are seen on Mars are led by their commanders as a splinter from the Cabal Empire. When commanders disobey their leaders, they are exiled. Most troops under the command of that commander voluntarily leave as well as their loyalty to that commander is unquestioned (broken legion grimoire, for example). The fact is, we haven't seen the Cabal emperor and the actual imperial forces he commands. Valus Tra'ug's grimoire states that Tra'ug wanted to invade The Reef and claim it for the Empire. However, when he took his plan to his superiors and they told him "no", Tra'ug declared himself independent and his legion decided to attack The Reef without the Empire's support, which of course backfired (he was caught). If the emperor of the Cabal ever decided to make a move for the Sol system, guardians would face an overwhelming force of Cabal siege weapons, including the ability to destroy entire planets and moons. The Cabal could quite literally "launch" Phobos at Earth and destroy us and the Traveler at the same time. At the end of the day, we don't know the motivations and goals of the Cabal Empire with regards to the Traveler - they've never specified in the grimoire that they want it or have a desire to simply destroy it.