I believe most politicians act in the interest of those who will help get them re-elected. Therefore indirectly they do serve the people and donors in their immediate community/region, city, or state.
The office of president is a lot more difficult to serve the interest of the people. Firstly in any given election about half of the people voted against you! The only thing they want is for you to leave office.
They never supported your party's platform agenda or its goals!
No matter what you do a large segment of people are going to dislike it. Therefore it becomes a game of keeping your (party leadership) people happy and playing the (electoral map).
You naturally want initiatives that retain your blue or red states along with offering something to those "swing states".
Everyone wants to be reelected!
Oftentimes it's during a president's second term that they attempt to spread their wings. Nevertheless most laws are enacted by congress so in order get anything done his/her goals has to benefit those who would vote in favor of it. Which explains how "pork" often gets added to bills. Essentially it's sort of a bribe/offering to Senator or congressman's state in exchange for their vote on a national bill.
That Senator can now go back to their state to remind people they brought so much money back home for projects and the like....etc