The short answer
Critical issues are often discovered late, during conveyancing, mortgage valuation, or surveys, when costs have already been incurred and timelines are tight.
The problem usually isn't that a serious issue exists; it's that it's found at the worst possible point in the process. By the time it surfaces, both sides have spent money and time they can't get back, which makes walking away far more painful than it needed to be.
Why this matters for your purchase
This is the kind of thing a PropM8 Property Intelligence Report is built to surface early, before solicitors, surveyors or lenders are involved, while it still costs nothing to walk away.