Lemmy is still small. The dev can't rely on tens of thousands of people on lemmy to buy Sync for $1-2 each like he could on reddit. The cost must be spread out on fewer users.
Well. I think it kinda works the other way around, I don't think many people are going to spend a whopping $20 for an app to use a social media platform that's not really taking off yet
You don't have to spend $20. You can use it for free without limitations. Or you can choose to spend $20 to remove ads if you like the app and don't like ads.