We know that in the Mortal Engines series, some cities are mining cities, doing exactly what it sounds like: mining the materials that their city needs for fuel, and so forth, instead of cannibalizing other cities. This makes sense: cities have enough energy to move themselves, so they should have more than enough 1 to accomplish the task of mining.
Why don't they all do this, though? It seems like mining is more lucrative and safer than chasing after and fighting a handful of small cities for materials, which could also lead to material loss during the combat.
1: Specifically, the kinetic friction force is just the gravitational force multiplied by the coefficient of static friction (because treads). The work is this force times distance. The static friction, let's say, is similar to rubber on concrete (0.5). Thus, roughly speaking, moving the mass of a city a certain distance (say 100 km) takes as much energy as would be needed to lift an equivalent mass half the distance (50 km). In reality, excavation of useless material will cause the energy needs to be much greater, but this doesn't matter much (if 2% of excavated material is usable, bringing a full city's mass of it up from 1 km underground is on the same order as moving the city 100 km over land). There will be extraction and reworking costs, but there will also be such costs with consuming smaller cities.