tl;dr, Why can't I war with Turkey for Istanbul, or Romania for its Oil Refinery strat. resource?
Because it takes a lot of time and works to add every bit of ahistorical land grab that the players can think of.
Follow-up: if I puppet Romania, will it give me the benefit of the Oil Refinery?
You don't need to annex or puppet someone to get access to strategic resources. They just have to be alligned close to you and be on the same continent or bordering the same ocean.
you can edit in acquire territory wargoals but I forget where it is and it requires some magically leaps of knowing commands and variables that aren't listed in the file you edit in.
You just need to add another region to regions.txt and add a "peace" before putting the province IDs in. Getting the correct IDs is the worst part if you're not using a tool like HoI3PosEd.