Part of the answer is that some of these services need to be scaled horizontally to be able to handle a significant number of users (e.g. tile servers, the core server), another part of the answer is architectural constraints (e.g. the core server needs to keep quite a bit of per-infrastructure data in RAM).

(Of course, it's completely possible to build a single container which runs all of the services in parallel, but then monitoring/scaling/availability/etc are more difficult to handle.)