Why resource planning almost always breaks at the same point
The problem is not the absence of a tool: nearly every firm has one. The problem is that availability is maintained by hand, somewhere other than where the data that determines it is created.
A contract ending, a project extended, half a day of training, a consultant returning early: all of these change availability and all of them happen elsewhere. As long as someone has to transcribe them, the sheet is a few days stale by construction, and trust in that sheet erodes until people stop looking at it and start phoning each other again.
The only stable fix is to calculate availability from operational data rather than maintaining it in parallel. That is why staffing, contracts and timesheets need to live in the same system, not in three systems joined by an export.