Because there are many different implementations, with different trade-offs and design decisions. Also, the implementation-specific documentation is typically kept fairly close to the code, as the docs need to be kept in sync.

So for example if you need to know how indexes in PostgreSQL work, look into the READMEs in the proper directory:

b-tree indexes: https://github.com/postgres/postgres/blob/master/src/backend...

gin: https://github.com/postgres/postgres/blob/master/src/backend...

brin: https://github.com/postgres/postgres/blob/master/src/backend...

etc. The READMEs also include links to related papers etc.