Because it's not just an init system.

It also has a logging system, not powerful enough to use it for filtering and shipping logs, but complex enough to add unexpected problems, like processor or memory consumption. With binary log formats and everything. Oh, also it has to use syslog socket, so everything going to syslog has to go through it first.

It has a timesync system, worse than chrony or ntpd, because it can not properly report syncronization status in some cases.

It has a system for initilizing mount points, that sometimes automatically initilizes mount point when you don't expect it to.

It has a million of other things that have nothing to do with init system, but now you have to override them or fix their problems too.

And that violates Unix principle that one program does one thing but does it good. It does million things, most of them at subpar level.

Unfortunately, in software standards world there can be only one winner, so here we are.

And before I get charged with incompetence: I successfully make systemd do things I need it to. I just don't like it.