Because (1) anyone serious about such things would want to minimize the likelihood of their desires being misremembered or misinterpreted, and written statements are much better in that regard than oral statements; (2) One of the key tests in deciding whether an oral declaration should be accepted is whether it's plausible that supposed witnesses are making it up. The sooner a statement is recorded in tangible form after its supposed utterance, the less plausible it would be for it to be simply invented. If a witness to an oral declaration should transcribe it in tangible form as soon as practical, why not just have the declarant himself do it, if practical?
2) Grampa, worth $20 million, writes on a scrap of paper that he wants to live no matter what the cost. How long would it take the family's attorney, being paid 30%, to invalidate it? 10 minutes?
On what basis would you suggest that it be invalidated?
That'll cost you 30%.
I'm no lawyer. I'd guess a case could be made that a) it was done in haste, b) not a clear mind (why would a wealthy man write on a scrap of paper?), c) he has some obligation to his family (he's brain dead and they're in poverty).
Hell, lawyers have broken iron-clad wills.