The general criteria for authorship require including the people who worked on the experiments and data for the paper, which can be more important contribution than most of the text in that paper. In other experimental fields, there are papers with dozens or even hundreds of authors, because it can take many people to get to a measurement of a single number in the paper.

Thanks, this is the bit I've been missing.

I raise you the Gemini paper https://arxiv.org/abs/2312.11805

All-in with the Foldit paper [1,2].

[1]: https://en.wikipedia.org/wiki/Foldit

[2]: https://www.nature.com/articles/nature09304

Goes to show how much money is being poured into this stuff.

For a serious answer, this is how it works in my field A researcher gets a grant with 3-7 co-investigators. This generates a bunch of data and other resources that will support 10 or more papers. Coinvestigators and PIs will ask their postdocs and grad students to write up a paper. PIs and co-Is go on every paper...because it's a paper from their grant. Then the 1 to 4 grad students and post-docs go on the paper, depending on their specific material contributions to the work, be it analysis, conception, or execution, or writing. The numbers can stack up.

Wait until you learn that the paper on the LHC has more than 5000 authors: https://www.nature.com/articles/nature.2015.17567