Because Arsenal suppose have a goal in that first half, but due to the pattern of playing it was not possible even the way they play against Real Madrid by beating them 5:1 not body will excepted that PSG can able to lead Arsenal by 1 goal right from the 4 minute match.
Merino in the 50th minute actually managed to put the ball into the PSG goal. However, based on the VAR review, the goal was disallowed and then declared no goal. This gave disappointment to the players, but this is what happened. This happened because Trossard was offside.