Gnome feels very anti Unix philosophy.
Either way, many package managers provide groups or patterns to make it easier for an end user to maintain their system. Pacman does have such a feature.
Look, for example, do you prefer to be able to install SDDM just because you like the themeing capabilities, and be able to do so, with just the bare minimum packages; or, install GDM and having to pull the whole GNOME DE as a requirement?
It's purpose is to provide and manage account information for applications that need it, so the user doesn't have to configure their email, calendar, whatever, in each application separately. With each application storing passwords in a who knows where, or each implementing their Gmail OAuth.
So it is quite logical, that application that uses accounts will also need gnome-online-accounts.
'Do one thing and do it well` -- how is that anti Unix philosophy?
GNOME has a keyring system service for storing passwords, so it's not necessary to get the entire desktop shell to access the keyring.
GOA is for storing account information: e.g. you configure your Gmail/nextcloud/whatever account once, and then all apps that need it can access it. So your Geary, Contacts and Calendar (separate applications) do not need each to implement account management and account specific auth flows -- not just passwords, OAuth is all the rage nowadays. And I, as the user, do not have to keep tokens in each app current.
It is also not 'entire gnome shell`. Just the required components.
here the required dependency list for control center on arch: accountsservice bolt colord-gtk4 cups-pk-helper gcr gnome-bluetooth-3.0 gnome-color-manager gnome-desktop-4 gnome-online-accounts gnome-settings-daemon gnome-shell gsettings-desktop-schemas gsound gtk4 libadwaita libgnomekbd libgtop libgudev libibus libmalcontent libmm-glib libnma-gtk4 libpwquality smbclient sound-theme-freedesktop udisks2 upower
while it may follow the letter of the philosophy is certainly doesn't feel like it's following the intent of it. the components form a sort of cyclic graph, where technically you can install any component but soon you'll find that you need all the others with no viable alternative component that will work reasonably well.
using these component outside of the DE feel like being a second class citizen that will be kicked out at any update, and then you'll have to scavenge for information from unofficial sources and the source code for what has changed that broke your setup.
Applications which are part of GNOME itself probably try to remain small and integrate well into GNOME e.g. Nautilus and Evolution. Applications which aren’t part of GNOME like Gimp or Geeqie rely only Gtk.
It just hurts if you want use a integrated GNOME-Application with a sole window-manager. That makes sense and we shouldn’t blame the developers for modularity and re-using.