I especially find it frustrating that many PMs think that the instant a system goes into production, the project is finished and everyone should be putting tools down and walking away. In reality, the post-go-live support can take a surprising amount of time, but is rarely accounted for.
One particular customer, a telco, had PMs that started in the telephony space and were especially stuck in this mindset. Imagine digging a trench and laying down some fibre for a customer. The second the hole is filled and the fibre is lit up, the project is done. There is zero ongoing maintenance! Fibre doesn't need monthly patching, or regular replacement. It lasts decades. Sure, it might be cut by a backhoe, but that's break-fix, not project work.
Now picture the same mentality applied to general IT projects. Build it, mark the project as closed, and walk away. Never patch anything. Never upgrade. Never consolidate. Just leave things precisely as-built forever or until it breaks...
... or gets hacked and makes national headlines.