PSG didn't wait that much before they signed Donnarumma.
Huge difference.
Juve are broke and not financed by an endless supply of cash like PSG. Besides, like
@Ronn said, they didn’t have to negotiate with Milan. In fact, every team that tried to negotiate with Milan while Donnarumma was on contract, failed to reached an agreement.
If Locatelli had an expiring contract with Sassuolo, he’s already be a Juve player by now.