mechmouse: Because they can't. 
 Games sold via GoG, Epic, Steam have their prices set by the publisher, a permanent or prolonged price cut for one platform would break competition laws in most countries. 
 Spectre: Some games here were cheaper on Steam. 
 This is an interesting debate. I pay here using dollars. Steam supports my local currency. However, many companies had a very different understanding of what "one dollar" means in my local currency. There's a range they seemed to have picked, and companies like Bethesda and Activision went for the highest value possible. However, many games do seem to be cheaper on here than on Steam... To the point that paying using dollars is way, way more convenient than using local currency. Even with sales. It's kinda crazy.