This is pretty fundamental stuff. You can't run a program if the disk that it's sitting on hasn't been made accessible yet. You can't start a network service that listens on a certain IP address if that IP address hasn't been configured yet, or if the destination for service logs isn't ready, or if our service needs to talk to some other service like a database which isn't running yet. Etc etc. Booting up a modern system to a graphical environment requires hundreds of things to happen in the right order.
As for everything else, the project announcement blog post from 2010 does a pretty great job summing up the shortcomings of doing "the simplest possible thing": https://0pointer.de/blog/projects/systemd.html . You can certainly still do things that way if you want, but most people value the benefits gained from a more intricate approach.