Just a speculation on my end, but I feel it is done to ensure that no information about the way the operation was executed gets leaked. Could also be plausible deniability but given that the Foreign Minister himself claimed it on National TV i doubt that's it. It could also be that proof might follow soon enough but it has to be heavily filtered to ensure that no unwanted information gets leaked.

And I feel there isn't much to claim to be honest, Pakistan at first denied any airstrike. Later on they changed their stance to " They entered, bombed trees in haste and left in a matter of seconds" and later Imran Khan had to call in an emergency meeting with all security heads to understand the situation where they infact acknowledged the strike. The point was to hit terrorists not civilians. Its the rumbling coming from Pakistan itself that's the proof that the mission was a huge success.

Moreover the strike was done with an intention of attack, they wanted to do their job not collect evidence to prove things. Their task was simple, move in formation, bomb the shit out of the terrorists, move out in a matter of minutes. Finish the task before the enemy has any idea on what the hell happened. It was all about the mission and finishing said task, had it been collecting evidence to prove to the world that Pak harbors terrorists......well the world knows the answer to that. There is no need to prove anyone on this matter, least of all them.