Why do some economic goals are incompatible with each other?

Most economic goals rely on their theorys. Most of the theories rely with the phrase "ceteris paribus" this means, all remains the same. This means that for economic theories to be acomplished, all other factors outside the control of the economist must be remain the same, however, it is impossible to control and keep the same a world with endless factors which are continuingly changing. This is why it is said that economic theories do not apply to reality. Hence, it is difficult to meet economic goals since there are so many factors that can change everything