Why are election campaigns in the US so long and painful?

Because it takes time to sling that much mud.

Because the longer it takes the more money there is to be made.

I agree with @Judi that money is a big part of it.

It isn’t that there’s money to be made. The duration is lengthy so that the piles of money have time to distort the facts and corrupt the process. As with most procedures in the country, pervasive ignorance assures that with enough money you can convince the masses of anything. A

All those people running the campaigns are making money. The media reporting the polls, televising debates, and talking about the candidates are making money. People making ads are making money.

So far it seems there’s little enthusiasm for these lengthy campaigns. So why do they still get so much attention so early? Why doesn’t the electorate respond with a resounding “meh” when candidates organise rallies so early on? Do people enjoy the circus?

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.

The system was designed before mass media as we know it.

The system was designed before mass media as we know it, and needs major reform.