Because Modern Warfare is published by Activision, which is a part of the bigger Activision/Blizzard company. So to save money, they just put it out on the Blizzard launcher (Battle . net) instead of having to pay Steam to sell it here and they keep a part of the profits.
Subway Surfers is an endless runner mobile game which is co-developed by Kiloo and SYBO Games, private companies based in Denmark. It is available on Android, iOS, HarmonyOS, Amazon Fire Tablet, and Windows Phone platforms and uses the Unity game engine.