Why are some chess championships defended while others have the champion starting from scratch?

Question

For example, the World Chess Championship as of the past few cycles had a candidates tournament to determine who faces the previous champion. Same with the Women's World Chess Championship (at least as of recently).

These are unlike other championships like say World Blitz, World Rapid, US Chess Championship, British Chess Championship, the women's versions of the preceding and the Women's World Chess Championship (until recently). In these latter cases, the champions start from scratch, so I find it weird to say Wesley So successfully 'defended' the US Chess Championship in 2021 or that Jovanka Houska didn't 'defend' the British Women's Chess Championship in 2021.

I don't even see why they are called championships in the 1st place instead of just 'tournaments' like say the Grand Chess Tour or Tata Steel. So, I don't see why we are calling MVL the 'current' world blitz 'champion' instead of the 'most recent' World Blitz 'winner'.

Or alternatively, I don't even see why we don't call Wesley So the 'current' Grand Chess Tour 'champion' instead of the most recent grand chess tour 'winner'.

Guess: A championship, as opposed to merely a tournament, implies some kind of 'representation'.

By representation, I mean representing

• a country like the US or the Philippines or the Commonwealth of Nations,
• all of chess,
• all of women's chess,
• all of blitz chess,
• all of women's rapid chess, or
• all of 960 (at least in slow rapid and lower time formats).
  But there's no representation meant by 'Grand Chess' or 'Tata Steel'.

Note 1: I'm not familiar with sports/gaming in general, so perhaps this can be or is answered by how sports/gaming in general work. You can answer chess-specific or generally.

Note 2: I guess we exclude talking about cases where we have to determine an inaugural champion, like say the inaugural FIDE World 960 Championship in 2019. Also, I guess determining a new champion if the old champion forfeits.

Answer 1

Why are some chess championships defended while others have the champion starting from scratch?

The simple answer is "History". The open chess world championship predates FIDE by many years and was already established as somehow the "property" of the current holder. To challenge the holder you had to be a good player, not necessarily the best of the challengers, and to have enough money to bet against the holder.

This tradition was not just a chess tradition. It came from martial arts like boxing which had pretty much the same sort of structure.

FIDE inherited this structure, as it were, and just continued it. When a women's world championship was instituted in the interests of women's equality this tradition was carried over to the women.

However in titles like U18 world champion it just makes no sense. The winner of the U18 championships 2 years ago often no longer qualifies because he or she is now over 18.

When it comes to continental and national championships there is not the same tradition as the open world championship. These have evolved in a fairer way with all-play-alls for smaller fields with pre-selection or the more modern Swiss style tournaments which allow much bigger fields.

That a round robin tournament is the fairest way of finding the best player is now more widely accepted. That is the way the candidates tournament is run to select the challenger for the world champion. It wasn't always so. When Fischer qualified to play Spassky in 1972 he did so the hard way by winning a short series of knockout candidate matches.