A large part of this is because the US – unlike almost every other country – does not have the same sort of multi-party, prime minister, coalition-based governing structure that everyone else days.
Take just about every country in Europe (and canada, and Asia, etc.). They are democratic and have elections, but they vote for a slate of candidates based on their party. They party with the most seats won is the winner, but they need more than 50% of the seats to govern. So the nature of the government is that coalitions agreements must be made and compromises are required.
Even more important, the leader is the Prime Minister – who is a member of their congress or parliament, and not a separate branch of government like in the US.
And in particular, all of these European and Asian governments are vulnerable to a vote of No Confidence by their parliaments – governments can fall when they are out of touch with their constituents.
The US is entirely different.
We have fixed terms (4 years) for our presidents. There is no such thing as a Prime Minister (the Speaker of the House is nowhere near as powerful). And there is no such thing as a ‘vote of no confidence’—all we have is the vote of impeachment, and you need good reasons to use that.
So… the real answer to your question is that the US governmental structure, for better or worse with its predictable 4-year cycles, has built in eternal campaigns.