I would agree with that in principle, but the sheer volume of accusers (hundreds of people telling police they were abused by Savile) makes an extremely compelling case. Even if 50% of them were lying, it would still mean the other 50% were telling the truth.
Only if he were alive, and then you'd be right - it would be dynamite.
But the question was, 'why do some people want evidence other than witness allegations'. Answer = because he's dead, because the witness allegations can't be tested, because there is money and attention in it............... etc.