Here's why:

* Genetic Variation: Humans are incredibly diverse, with billions of years of evolution and countless genetic mutations shaping our population. This means there's a huge amount of variation in our DNA, with many different alleles (versions of genes) circulating within the species.

* Evolution is Ongoing: Evolution doesn't stop. Even if an allele becomes very common, it can still change in frequency over time due to factors like genetic drift, natural selection, and new mutations.

* "Fixed" is Relative: The concept of a "fixed" allele is often used in the context of specific populations. An allele might be fixed within a small, isolated population, meaning everyone in that population has the same version of that gene. But even then, the allele could be different from the version found in other populations.

What About "Nearly Fixed" Alleles?

While there are no truly fixed alleles, some alleles are extremely common and nearly universally present in the human population. These might be considered "nearly fixed". Examples include:

* ABO Blood Group: The ABO blood group system has a few alleles that are very common, though not universal.

* Specific Genes: Some genes, particularly those involved in essential biological functions, might have very few variations within the human population.

Important Note: The concept of fixed alleles is a simplification. Even within a single individual, there can be slight variations in their genetic material due to processes like mosaicism.

In Summary:

While we can talk about alleles being very common or nearly fixed in certain populations, there are no truly "fixed" alleles in the entire human species due to the ongoing nature of evolution and the vast genetic diversity among individuals.