The single gene crosses you have met are useful, but they are the exception. Only a few features are controlled by one gene. Most phenotypic features are the result of multiple genes working together, rather than a single gene.
Human height and skin colour are examples. Many genes each have a small effect, and their effects add up. Because so many allele combinations are possible, there is a whole range of phenotypes between the extremes. This is why features like height and weight show continuous variation, with every value in between, instead of falling into two neat groups such as tall or short.
The genome interacts with the environment too. Your genes set a limit for your height, but diet and health as a child also affect how tall you grow. A Punnett square for one gene therefore cannot predict a feature controlled by many genes.
