Because it wasn't. No pull, no foul. He let go before the ball came in. Nowhere near enough for a foul.
It was a flap from Donnarumma and he tried to claim a ridiculous foul to cover up his error. Glad to say it failed. Just bad keeping.
I agree he flapped but the player had hold of his arm whilst the ball was in flight. I don’t see how if you hold the keepers arm whilst the ball is in play, which affects his ability to move freely, isn’t a foul. Not that I care so much, just think it was a foul.
For me, it had no impact on Donnarumma because he had a free effort to go for the ball and just flapped at it. It would have been very soft for me and certainly not enough for VAR to intervene.