Why can multiple alleles result in many different phenotypes for a trait

Say you have ghjL, this is a genotype, but there are several options because each letter has a possibility of being dominant. Each allele represents a different phenotype. Example: G is green eyes, g is brown eyes. H is blonde hair, h is brown hair. J is light skin, j is dark skin. L is long toes, l is short toes. Etc

Sorry for the long response, hope this helps. :}