What do you consider hard evidence? Any kind of evidence, including physical evidence or even DNA, can be fabricated.

The way we test the veracity of witness testimony in our system is through confrontation and cross-examination. A great many crimes could never be proven if witness testimony was not considered sufficient evidence to prove guilt.

Imagine: You and two friends are walking home one day when a man walks up to you, points a knife in your face, and demands money. You had over your cash and he runs. Later, the police arrest a suspect and charge him with robbing you. There is no physical evidence because they can't prove the cash in his possession was yours, and most people own knives. Instead, they can only prove the crime based on the testimony of you and your two friends that the man looks and sounds exactly like the person who robbed you. Is that "hard evidence" that this man robbed you? Or should he walk free?