The short answer
Late discoveries leave little time to resolve problems, leading buyers, sellers, or lenders to walk away.
The closer to exchange a problem is found, the fewer options everyone has to fix it. What might have been a manageable issue with weeks to address it becomes a reason to abandon the whole transaction with days left.
Why this matters for your purchase
This is the kind of thing a PropM8 Scan is built to show you early, before solicitors, surveyors or lenders are involved, while walking away still costs nothing.