It depends on each country and region, but in the end of the day, politicians need to play the political game to maintain power in order to do whatever. It doesn't matter the political system. It's not just doing good to the public or to their self-interest, politicians need that power and need to know where the power comes from.
They need to balance self-interest/will to seek power, geopolitical interests, their voting base (if it applies), their allies, each faction who holds power in each sector in society, and finally the common folk. Sometimes these interests will match somewhat well, but usually they are conflicting. The bigger the government the more conflicting will be each faction's interests.
Usually the common folk is very uninterested/ignorant of politics, so politicians don't need to cater their needs besides promises, propaganda, or fear-mongering. Individually the common folk has very little power to do something. United and organized they may be able to make demands.
There are some few examples that politicians in power may have the best interest and/or have a good intentioned vision for the nation. Of course, it may not match the will of the people because let's be frank here, people are selfish pricks and each one has a different idea of what is good for the nation.
Every baffling or damaging decision done by a politician will have a benefactor. If he/she is doing it anyway it's because the other conflicting sectors of society have less saying/power to oppose it. In the end of the day, politicians have to return the favor to those who give them power.