That’s because Darwin’s is an observation of self-replicating systems in general and not an abstract idea or new concept. It’s tautological that the most successful self-replicating systems are the ones most successful at self-replicating. Fitness is by definition being successful at dealing with the environment in regards to self-replication. Darwin’s in the general sense is in my opinion not a theory but a fundamental observation of what is. The only theory is that there’s a continuous lineage of living systems that evolved one into one other. But that’s more of a teleological issue and can be understood as a theory subject to rejection.
Darwin’s theory is that the diversity of life, the nested hierarchy of traits noted by taxonomists before him, can be explained by that fact about replicating systems — and doesn’t require more than that and mutation.
Darwin explained an open question (“why do taxonomies look like this?”) by relating it to fitness and to trait changes via artificial selection (eg, breeding horses and dogs). That’s why it’s about how finches developed traits in nature — because repeated fitness selection in nature like farmers breeding lineages could account for differences in animals.
That the “fit survive” isn’t the theory — just a fact about the world.
> That the “fit survive” isn’t the theory — just a fact about the world.
In Darwin's time the idea that "the fit survive" was very much a theory.
A competing idea that we might call "the just survive" was the basis of trial by ordeal and trial by combat, of prayers for healing and rescue, and of holy wars. Many people still believe that God chooses who survives, and thus do not accept survival of the fittest.
“Survival of the fittest” is a statistical observation of a population ensemble. You can be otherwise fit and eager to replicate and be hit by a truck. Or you can be sick and almost infertile but got someone pregnant. So Darwin doesn’t explain the human condition and why the just might fall due to sickness or the wicked become rich and prolific. That only Heaven knows.
