Actually, a better answer than the one I originally thought of, is that the Dust is responsible for their world splitting (in a way which causes it to increase in measure). So worlds which permanently defeat the Dust don’t increase in measure past that point, worlds in which the Dust takes over don’t summon anyone, and hence the majority of measure in worlds that can still perform summonings are those worlds which have survived but not yet defeated the Dust.
Actually, a better answer than the one I originally thought of, is that the Dust is responsible for their world splitting (in a way which causes it to increase in measure). So worlds which permanently defeat the Dust don’t increase in measure past that point, worlds in which the Dust takes over don’t summon anyone, and hence the majority of measure in worlds that can still perform summonings are those worlds which have survived but not yet defeated the Dust.