Most of these services have a dependency on the service manager and can't run without it.
For other services, such as journald and udevd, the dependency also works in reverse and the service manager can't run without the services.
In many cases, it's not entirely clear why they depend on the service manager itself. logind in particular was moved into systemd “in anticipation of the single writer cgroup architecture", an architecture that never came to be because it was quite clearly a very bad idea, and then elogind was forked to separate it again with no loss of functionality.
There is no reason for logind to exist, elogind should be the only thing that exists as it does the same thing without depending on systemd; elogind can also be used in conjunction with sytemd with no loss of functionality.
Even stranger things happen, such as DBus performing activation viā systemd by use of a private, nonsstandard a.p.i. for which there are standardized protocols specified by Linux-base that systemd also supports. That they chose to use a specific unstable, undocumented a.p.i. rather than a standardized one to do this is certainly a political rather than technical decision to create a dependency for it's own sake.
And that is indeed what many RedHat projects have done over the years; they have created dependencies on each other of little to no technical merit as form of product tying to encourage adoption of more RedHat software. — this is certainly not limited to sytemd.