On the one hand, it makes sense, on the other, it doesn't. Because in the end, MAs have a much longer life expectancy than mortals. It doesn't make sense for IE to feed on life force if in the end MAs end up living longer.
My theory is that it's purely biological. You see, MAs undergo Rebirth when they enter a new "Great Realm", and Rebirth reshapes practically the entire body; perhaps this new individual's biology is already programmed with a trade-off: Investing in longevity (maintaining and repairing the body) could mean fewer resources available for reproduction (producing lots of offspring). In other words, the new bodies of the MAs already "know" that they don't need to rush to continue the lineage, because they have time.
And this may be the reason why MAs live longer than cultivators, as cultivators don't go through Rebirth (at least not in the initial realms, and at least MC hasn't so far) their bodies don't undergo a complete change in their biology, they just have their bodies (soul and mind too?) strengthened, which increases their lifespan, but not drastically, even though they are superior to MAs in almost every other aspect.
Edit: Unfortunately, the difficulty of early-stage cultivators to reproduce cannot be explained as anything other than "the will of the Heavens", at least not in my eyes.