There are many reasons why Goku could beat Saitama, as well as why Saitama could defeat Goku. Being from two arbitrary fictional worlds, and having no definite way to measure not only ability but most importantly their potential, both are subject to a fan’s understood preference. The reason for this is that both characters seem to have limitless potential. Very simply, Goku can continue to train and become more godlike, as well as utilize his dragon balls as a backup. For Saitama, his power is already beyond conventional measurement, and being a limitless individual, there is very little capping to his potential. As such, both forces are difficult to determine and would likely just evolve into an explosive stalemate with one knocking the other over until the other overcomes and vice versa.