Part of it is that EU countries charge VAT on digital sales and prices must include it, while in the US there’s sales tax instead, which depends on the state and is only added at checkout time (and also often lower or even zero for digital goods). For example if I pay 80 Euro in Germany, then Steam gets to keep 67 because those 80 Euro include 19% VAT.

The other part is that in a market economy, prices are determined by what generates the most profit, and not necessarily related to cost or based on fairness. EU customers apparently pay 80 Euro often enough that it makes more money than 60 or 100 Euro so developers set their price to 80 Euro.