Did they not explicitly state in the Constitution that it isn’t allowed because they thought it was ok or because the idea of that happening was too ridiculous to warrant their attention?
When we look to the framer's of the constitution, the main debate was whether the pardon prerogative should extend to "treason" (it does) or "impeachment" (it doesn't).
Hamilton writes about the pardon in paper 74:
Humanity and good policy conspire to dictate, that the benign prerogative of pardoning should be as little as possible fettered or embarrassed. The criminal code of every country partakes so much of necessary severity, that without an easy access to exceptions in favor of unfortunate guilt, justice would wear a countenance too sanguinary and cruel.
Here we can see something of the 18th century mind on pardons. They are a mercy by which cases of "unfortunate guilt" can be mitigated.
Hamilton goes on to argue why this power should be in the hand of one person. As a committee will tend to follow rules, whereas one man can act out of shared humanity. The act of pardoning is intended to be one man showing mercy to his fellow creatures, who have reached a state of guilt as much through bad luck as through evil intent, and who deserve release from the punishment (which for many crimes would have been capital)
one man appears to be a more eligible dispenser of the mercy of government, than a body of men.
While not explictly stating it is impossible, nothing in these texts suggests that self pardoning is intended, as self pardoning is not an act of mercy, but self-preservation.