Well, it's a particular use case.

#Alexandria will probably also have a database, for some premium services (self-hosted or rented), but I don't want someone needing a database to use the basic app functionality. That's sort of overengineering it.

Running a database uses more resources and is more complex than just running a relay with a stringent whitelist, or rolling events off according to a deletion-schedule, or something.

Npubs could simply download events to cache or have a SQlite or whatnot DB, or a relay on localhost.