But a lot of Western Roman generals did rebel, or perhaps it's not called rebellion for the ones that succeeded in getting the Senate to acknowledge them as Imperator even if they often didn't live long after that. And they're the same people who would be leading the themes if the Western Empire used them.
This also occurred in the east during the period of the late [combined] empire. However those generals were basically military appointments who didn't directly associated with or have interests in the region they were stationed in.
As I understand it with the themes the key was that forces served locally and hence had knowledge of and an additional incentive to defend the region. The basic idea would be that they are locals used to defend their territory and are supported by the core regular forces coming from the capital regions when required. IF and I admit if is the key question you can build up such forces their less likely to rebel. Both because they have less interest in seeking to claim the imperial throne and because any attempt to do so would involve deserting their home base. Even if the general decided to make a bid for the purple the bulk of the force he commanded would have less interest in supporting such actions.
There were generals who gained considerable power and in some cases established dynasties in Byzantium but I was under the impress they largely used imperial rather than themic forces for such moves to seize power. Could be wrong here so would be glad to find out more.