Why can my computer run the witcher 3 just fine but fallout 3

>>384532028
Memory allocation problems, poor engineering decisions made in various areas and some strange hacky solutions to problems that have no right to exist which don't let your video card do as much lifting as it should while making aftermarket fixes hard to apply, if not outright impossible.

Some basic tweaks that might bring performance up:
-Lower God rays (this is especially helpful if you're not using an Nvidia card)
-Lower shadows (for some Godforsaken reason they run the shadow calculations through CPU Core 1 instead of your GPU, which results in bogged down performance in any context where there's multiple shadow maps running at once).
-Try ENBoost
-There's a few tweak guides out there but some of them can actually make performance worse.