Because it diminishes his global exposure, how many people watch a Barcelona vs Malaga or Barcelona vs Deportivo, compared to PSG vs Nice or PSG vs Rennes?.
You can see it in this same forum when people bring up young players doing great at 2nd tier leagues. "yeah, he's killing it at Montpellier in the Ligue 1, but is he good enough to do it at Arsenal level?", well that happens with stars too, he can score 35 goals this season, he's scoring them at almost the same level as Russian/Portuguese leagues (46k coefficient for France, 42k and 39k for Portugal).
God, even Zlatan scored 38 in his last PSG season and people had doubts he could pull 20 in the PL, that's how bad getting low exposure, or getting it for putting 4 against Caen hurts a footballer brand