A thing unique about our country is how important, and massive, our individual party primaries are. In many other countries, where there are multiple parties that are each representative of a single ideology, the nominees are chosen by a collection of party superiors, or in many cases, they are already the nominee by default for being the leader of the party. Our countries two party system results in the two parties being very big tents and catching all either left of center or right of center. This means that unlike in other countries, our primaries for each party are the true ideological battles. For example, in other countries, there is a progressive demsoc party and a more generic liberal party, each serving their own particular groups. In America, both of these groups fit within the democratic party, so the battle of these ideologies is represented in the primary, such as Sanders vs Clinton, or Sanders vs Biden. You have to take that into account when looking at the scope of our elections in comparison to other nations.
It still does not need to be as comically long as it is today. It would still work if the primary calendar was more condensed and the national conventions in late August.
I think announcing your campaign early is the only way for lesser known candidates to get into the conversation. I think that puts the whole field under pressure to announce their candidacy as early as possible and the media and the parties go along with it.
As Grassr00ts stated, the top candidacies are not that contested in other countries, or at least they don't make such a fuzz about it.
