Why are most laws created for political gain and reflect public sentiment?

That is very much what is discussed in Plato’s “Republic” in the form What is justice and  What would an ideal state look like?

One point of view is that most ruling classes create and enforce laws for their own benefit, whether they do so consciously or not. Given that modern states generally rule by a system of political parties, this amounts to saying that elected lawgivers create laws that benefit their parties. The German philosopher, Nietzsche, calls this a “master mentality.”

Another point of view is that the non-ruling classes want laws that protect them. They will support those lawgivers who create such laws. The German philosopher, Nietzsche, disagreeably calls this a “slave mentality.”

The French philosopher, Jean-Jacques Rousseau, in his book “The Social Contract” points out that most societies are an amalgam of these two forces. The people need a state to provide protection and services; in return, they are willing to lose some of their personal freedom by submitting to the laws of the state. A society will remain stable only as long as these two forces provide benefits to both political parties and the public.

Marx, of course, said that this happy state of affairs never occurred in nature, because the ruling classes were the employers and had absolute control of their employees. This is why Marx thought that the only outcome that would benefit the people would replace rule by political parties with direct rule by the people.