Is a large number of security holes not enough of a reasons?

In the resolver, in the ntp service, in the logging system, pretty much every part has had them and they keep coming. Largely because they introduce complexity that most do not understand. Some unfortunate decisions have not been recitified until the original developer lost interest and their heir had to make a heroic effort in figuring out the code.

Let's also not forget that the only time in history a major Linux distribution was backdoored, was only because some openssh package maintainers felt the need to link sshd to libsystemd. This was obviously not directly the fault of systemd maintainers but the culture around it absolutely contributed.