The problem is, there is almost always some "trivial" system (public web site, severely outdated wordpress blog, or worse) that some poor fool in marketing/product "HAD TO HAVE YESTERDAY". The admins knew it wasn't mission critical, and would only be "temporary". So they spent minimal effort to set it up, skipped over all of the process and security hardening they would do for a proper release, and left it.
Of course, we know what happens: some hacker finds the exploits, then pivots to explore the internal network.
You will find most big enterprise-y shops build networks with hard exteriors, and soft interiors. Very few of their security plans are capable of a threat from inside the network.