Because getting those upgrades is a boring grindathon of fetch quests and gathering resources for the obligatory crafting mechanic that isn't remotely worth the trouble when you can curb-stomp every enemy in the game using just the starter weapon. The only useful upgrade in the game is the "fast travel" skill because it lets you skip hours of traipsing around the (also obligatory) mind-numbingly boring open world, and beating the game without any upgrades makes it much shorter and actually marginally challenging. Also I paid for the game and I'll play it any way I want.
Why do you ask?