In Mortal Engines, why are there no "protector" cities?

Question

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?

Answer 1

London is relatively unique in the story given how far it is from its natural hunting grounds. By comparison, German-speaking cities are far more happy to ally and offer mutual protection. This evidently includes allowing smaller 'trading towns' to group around them.

And so the Traktionstadtsgesellschaft was born. The twelve great cities, swiftly joined by others, swore that they would eat no mobile town until the Green Storm was destroyed. They would survive instead by devouring Mossie ships and forts and static settlements until they had made the world safe again for Municipal Darwinism, which every civilized person knew was the most natural, sensible and fair way of life ever devised.

They turned, they fought, and they forced the startled Green Storm to a stalemate. Now, a broad ribbon of no-man’s-land wriggled across the Hunting Ground from the southern fringes of the Rustwater Marshes to the edges of the Ice Wastes, marking the boundary between two worlds. To the east of it the Green Storm were struggling to plant new static settlements and reclaim for their farmers land which had been ploughed up and polluted by centuries of Municipal Darwinism. To the west, life went on almost as before, with cities hunting towns and towns hunting villages; the only difference was that most mayors sent a portion of their catch to feed the Traktionstadts.

That being said, they recognise that once the immediate threat of the Green Storm is passed, they can return to Municipal Darwinism which is considered to be the appropriate and preferable state of being.

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You might also want to note that the idea of satellite towns has been tried before, but in the latter days of the traction era everything has become increasingly centralised. Towns do trade with London though, presumably this offers them some limited protection.

Whatever the town was, it was small, only a suburb really. Tom amused himself by trying to work out what it might be while Hester picked the lock on a hatchway and led him up a long stairwell with rusty walls that steamed in the heat from the engines. He thought it looked a bit like Crawley, or Purley Spokes, the suburbs that London had built back in the great old days when there was so much prey that cities could afford to build little satellite towns. If so, it might have its own merchant airships, licensed to trade with London.