Because he is already dead of old age in that timeline. They cant change that now. The Gravestone places him dead THERE at 86 years of age. The Doctor cant alter it, it would create another paradox. How would they know to go get him without the gravestone, but there would be no grave if they went and got him.
Except that all they and we know is there's a stone with their names on it. There's zero proof that they're under it. And even if there was, that just means they'd need to hold the funeral there.
It's no more a definite finality than the Doctor's gravestone in Revelation Of The Daleks was.