system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the American election campaign process take so long?

I'm Canadian, and in general, in Canada, an election is called and a month or so later, there is a vote.  The whole process -- campaign included takes about 2 months maybe... Actually, it's usually only one month.  I'm also living in South Korea right now where they impeached one president, and elected a new one yesterday... and that took about 2 months total.  But in the US, it goes on for 2 years.  Why is it so long?  From the outside, it seems so pointless.
gpt
From the inside it seems pointless too, to a large extent. But to answer your question, it's important to look at the schedule from the back to the front.  

1) Election Day is the first Tuesday after November 1 (meaning between Nov 2-8) every four years, like clockwork. The US does not have snap elections for the Presidency, so all campaign tradition, media, consultants, and expectations are premised on a calendar that is predictable and intentionally slow. 

2) To realistically be a candidate on Election Day, a person must be the nominee of either the Republican or Democratic parties. These nominees are officially chosen at that party's National Convention a few months prior to November. Since 1952 these have been held between early July and the first week of September. So, one can think of the *actual* Presidential Election as not officially starting until each Convention has selected their nominee. 

3) However, to become the nominee for their party, a person must receive the majority of votes from the attending delegates during their party's Convention. These delegates are sent by the party's apparatus in each state. These delegates are bound to vote at the Convention as dictated by the state-level party apparatus.

4) However, for a person to get the state party apparatus on their side (meaning, to get the delegates to vote for them instead of another candidate at the Convention) they must first demonstrate that the state party *should* get on their side through a show of strength and legitimacy. This is done one of two ways - by a direct vote of the party members across the state (a "Primary Election") or by localized arguments / debates / cajoling all across the state simultaneously one night on a precinct-by-precinct basis (a "Caucus"). Winning the Primary/Caucus for a state translates into delegates at the Convention. 

5) It didn't take long for some state apparatuses to realize that if they scheduled their primaries earlier than other states, they could receive disproportionate attention from candidates desperate to build momentum. States like Iowa and New Hampshire are notorious for going first so that every person who even thinks about running for President has to attend their state fairs and local barbecues, kiss the ass of every town mayor and PTA president, etc. This stereotypically results in promises to the Iowa farmer to protect their farm subsidy, etc., all because the candidates desperately want to show strength by winning or placing well in an early Primary. A poor showing in both Iowa and New Hampshire dooms many campaigns. Because of this disproportionate attention, other states started leapfrogging over Iowa and New Hampshire to make *their* Primaries more important. So Iowa and New Hampshire leapfrogged *them*, and this continued for some time until in recent years the National-level party organizations started imposing sanctions if states moved any earlier in the calendar. This became an issue in the 2008 Democratic Nomination race between Barack Obama and Hillary Clinton because Michigan and Florida moved their primaries too early in violation of Democratic rules. Because of all this leapfrogging, the first votes cast for the Presidential cycle are the Iowa caucuses held in January of the election year (so the next ones will be January 2020). 

6) Of course, those are the votes. Campaigning to win those early votes requires personnel, which requires money and expertise, both of which are in limited supply. So before the Primary campaigns even start there's also what's called the "invisible Primary", in which the candidates go around courting influential political figures, donors, etc building infrastructural and "insider" support to either get resources or send signals to rivals that entering the race will not be successful. Moving quickly to secure that support has advantages, even if it's done privately and not in media view. This can take a while, particularly for candidates who are not consensus strong candidates. As an example, Hillary Clinton's expected entry into the 2016 race absolutely stifled Democratic rivals in 2014 such that literally no registered Democrats with national standing ran against her in the entire race until Bernie Sanders, seeing no internal rival to Clinton, changed his party registration to run against her in an attempt to shape the party position. By contrast, the Republicans were completely fractious and factionalized through the entire Primary process because even though Donald Trump was repeatedly and consistently mauled by the opposition in early Primary after Primary, that opposition was split between a half-dozen legitimate contenders, none of whom could beat his 35/40% showing and they ate each other to death, each thinking they would be the last one standing to easily defeat him. By the time the rivals diminished, it was too late to overcome his delegate lead. Had the Republican Party been better managed and had better early candidates during the "invisible primary", the candidate pool would have been less fractious and Trump less likely to win.