Any act or process which can have a profound impact should not be at the discretion of just one person. Giving an elected official the power of the pardon as exercised by himself only is to bestow a prerogative of a monarch.
And we've seen from drumpf, in spite of the relatively few number of pardons he's issued, a distressing trend. That his concerns are essentially not with redressing injustice, but with rewarding friends and accomplices in the furtherance of his own protection from implication in his illegality.
In this respect has the power of the pardon shown to be open to egregious abuse.