Yeah people don’t know how to counter that. In low tiers, no one knows how to counter bastion either and it’s kinda funny.
Sometimes it's just that no one helps the person who's supposed to counter Phamercy. If the enemy has a little bit of game sense and know how to use cover, Mercy can easily drop down or hide behind building to avoid fire. Pharah gets healed up unless your hitscan can land consistent shots fast. But I'm gonna bet that everyone is just gonna blame the hitscan. Nobody ever thinks that maybe the hitscan needs pocketing as well, or that the shield can help the hitscan by providing protection against Pharah rocketing him. That's just one of the few things I noticed when I played on my smurf. It's super ridiculous how everyone is ready to be toxic to the one who tried to help the team to counter pick but refuses to help him.