Edit: As I'm reasonably certain there are no protector cities I'm rewording the question to ask whether there is a specific in-universe reason this strategy has not been pursued.
At the start of the Mortal Engines movie we see London chasing and eating Salthook, a small salt-mining town. Afterwards we know from dialogue that they have obtained enough fuel to last them a week, as well as some items such as an electric toaster. But they didn't get any salt (because Salthook dumped it during the chase) and they probably can't obtain it so easily in the future, because they just ate the town that specialised in producing it.
Suppose that instead of preying on towns like Salthook, London offered them protection from predators in return for payment in kind (Salthook after all was a trading town). Smaller settlements would be eager to align themselves with a protector and London would obtain resources they couldn't easily produce themselves, on a more sustainable basis than they would through predation.
This would seem like a viable solution to the problem of London and other big traction cities at the time the novels and movie are set: too many predators chasing ever smaller and fewer prey. It could be seen as a sort of "evolution", from municipal hunter-gathering to municipal "agriculture". So I'm wondering if there is a reason this strategy hasn't been tried?