The kernel is already monolithic enough without adding another piece of monolithic software that everything depends on. IMO the Unix philosophy means we should have interchangeable parts.
There's some amount of user error here but when I did use systemd I had a hard time turning off services I didn't want because they were in the wants-to-have entry of other services. It's like a separate config area to maintain with a specific maintenance tool software instead of flat files.
I'm unfortunately using distros with systemd now tho.