Key takeaway
A spreadsheet's real cost is the ongoing human labor of faking structure it was never built to hold.
"We'll just track it in a spreadsheet" is usually said early in a project, when the scope feels small enough that a proper tool seems like overkill. It is rarely said again once the project has grown past the point where one person can hold the whole structure in their head, but by then the spreadsheet is load bearing and nobody wants to migrate off it mid-project.
The cost nobody puts on the timeline
A spreadsheet has no idea that task B depends on task A, that a person is the owner of six things due the same week, or that a milestone just slipped because three of its dependencies did. All of that has to be maintained by hand, by someone, continuously, or it quietly goes stale. The cost is not the spreadsheet itself, it is the ongoing human effort of keeping a structure-free document pretending to represent a structured project.
That effort is easy to miss because it is spread across many small updates rather than one visible line item, but it adds up to real hours every week, usually the PM's.
The more expensive failure
Worse than the time cost is the decision made on stale information: a status reported as green because the spreadsheet said so, three days after it actually turned amber. A tool that keeps dependencies, ownership and status connected, instead of trusting someone to re-derive them by hand every time, does not just save time, it keeps decisions closer to what is actually true.