Why? Because you can never be sued for disclosing everything. But you can be sued for not disclosing as few as one thing. If you even have to ask your realtor whether you should or shouldn't disclose something, you almost certainly will have to disclose it.
Realtors loathe and fear too much disclosure because it can dead end sales before they even start and that is why you should do it. If telling the truth will affect the sale of the house, then you should consider fixing the problem (and still disclosing that you repaired it) or giving the buyer the option to purchase it with the known defect.
There is no such thing as too much disclosure. Only too little.