PSG deemed he was "too old". That's why they probably low-balled him, despite the fact that he was almost a legend there. If you worked with a company for many years, only for them to lowball you with lousy contracts in your 40s, would you still want to work for them or you'd move out to somewhere that people value you more?
So Cavani decided to "move on". Then within a group of AS Roma, Atletico Madrid and Manchester United, he chose the biggest company (And the biggest company paid him the most, befitting their status as one of the biggest companies in the world as well). If you were in Cavani's shoes, would you have made the same choice?