Because stage and close date are rep-authored opinions with no artifact behind them. Fix it by making progression artifact-driven — the document drives the stage — and by measuring slip: how many times a deal's close date has moved is a better predictor than the stage it sits in.
Add pipeline coverage and stage-aged views so a deal that has sat in negotiation for three cycles is visible without anyone having to ask. In consumption businesses the deeper fix is structural: forecasting usage is a data-science job, not a seller's, because reps cannot predict how customers will consume.
Every claim above comes from work LeanScale published. These are the sources.