Six fingered offspring are formed from normal parents, why?

Dominant' traits will actually disappear faster if they are disadvantageous.

Think about it: if everyone who has even a single copy of a particular allele is at a disadvantage (manifests the phenotype, in this case six fingers), then even single copies are selected against.

In the case of recessive traits, selection occurs only against homozygous carriers, who may be very rare if the allele itself is rare.

A concrete example would be something like Tay-Sachs disease. If the allele that causes this were dominant, every carrier would die before adulthood, and it would occur only as a very rare de novo mutation. But because it is recessive, it persists for now; heterozygous carriers have no disadvantage.