For instance, when King of Hammurabi formulated Code of Hammurabi, one of the oldest constitution, he made the laws to establish law and order in his empire as well as to spread his fame as the just King. His laws that classified humans into three distinct classes superiors, commoners and slaves was accepted as a fair law during that time and for many centuries to come. However, it is easy to see today that it violated the basic principle of law: equal treatment of all human beings in the eye of law and many others. Nevertheless, it was considered just for a very long period of time which proves not only powerful people can enforce laws for their own interest but also establish its acceptance in the society.
Likewise, men and women were considered unequal in almost all the cultures of the world until only a century ago. Women's equal participation in rights and duties is a very recent phenomenon. Women's status in society had always remained inferior in the past. If laws were made for the good of every member of the society women should have been recognised as equal from a long time ago. However, this did not happen which proves that laws were made by supposedly powerful for their own good.
Furthermore, a lot of minority groups had to struggle to establish their rights. For instance, same sex marriages got recognition only a couple of decades ago. The rights of disabled and the people from LGBTI community are still not recognised in many countries. These kinds of instances give us ample evidence to conclude that laws do not necessarily protect the interests of less powerful members of the society. Therefore, when we think that we have a very robust justice system in place in our century, in all likelihood, we might be doing injustice to a significant population of humanity.
Having said that, laws necessarily do not favour the powerful always. We have a great evidence that many oppressed people who sought out legal remedy have gained justice. For instance, poor people can take rich people to court and win if they successfully prove to have been victimised. Also, when they fail to get justice they have the option to go appeal in the higher level court for justice. Therefore, laws necessarily do not protect powerful members of the society.
In conclusion, laws in modern times are neither made by powerful nor favour the powerful blindly. Although history has ample examples of the prejudices and preferential treatments, modern time has come a long way from those times. While still there is some possibility that powerful people make the laws to protect their own interests, the power difference is decreasing and getting more dynamic. With humans becoming more and more wise, the laws are increasingly being made and enforced for the good of everyone.