Because the Ref is a cunt. Here's a short excerpt from the match report from FI:
Simone Verdi’s strike tested Buffon from a tight angle, then Matthijs de Ligt charged down another Verdi effort and it bounce off his thigh on to his arm. Usually, this would not be considered handball, but as De Ligt’s arm was moving upwards and detached from the body, the referee and VAR decided it did qualify. De Ligt was booked and will be suspended against Milan, while Belotti converted the penalty, as it had enough power to go in despite Buffon getting a hand to it