First, because foreign aid is a burden on tax payers. Secondly, it seems lately that America rescourses seem to be falling into the hands of organizations like ISIS. ISIS fighters are using our gear, our guns, and our supplies that we meant for Syrian Rebels. Stop feeding them and they will starve.
Ahh. Well as you said, it's not that simple. We had these really great visits when I was in college, from the State Department, and even Abba Eban. The thing is, over in the Middle East, everyone willing to sell anyone else out for any price. So you can never be sure of your allies or enemies. Sometimes the former are the latter, and vice versa. Same was true in Vietnam, actually.
So if you don't aid the allies who might turn around and sell/give your aid to your enemies to buy something from THEIR enemies, then you have more costs. This is why the US often ended up supporting dictatorships. Evil1 and Evil2.
In Syria, there have always been three religious factions: Sunni (=Catholic, Saudi), Shiite(=Pentecostal, Iraqi); and, Assad belongs to the third (smallest) the Alawites
. (Not sure what Christian analogy best applies to that sect.)
He got that dominance (via his dad, just after WWII) by balancing off the factions
. The rebels are among those. They are all untrustworthy. That's why we let Assad (and even helped him) stay in power so long, but then the Cold War was part of our motive.
As goes Syria, so goes Lebanon (Liban). So then you're looking at a geopolitical importance far bigger than Syria. Biblically, it's the same issue as applied during Seleucus and his progeny. All Daniel 11 stuff, really. That's the territory over which the Seleucids and the Ptolemies, often fought.
Russia had no better luck than we do; really sad record in Afghanistan. Gorbachev went to Baghdad to urge Hussein to fight against us. Pretending to us, to be our friend. All this, a short time before Dubya went in.
Ishmael's sons. That's basically the issue.
Larger picture is, these smaller countries use either US or Russia as leverage against their own little factions. That's the history of post-WWII Africa and India.
So we can't just withdraw. They will go to Russia/China and we know that. They ARE going to Russia/China and we know that. The Cold War never ended. So that ties back to what you said about Russia and China. Proxy countries, with us big Three fighting, and Europe just sits by, still angry over it being the pawn in WWII, waiting for which of us Three will go down first, hopefully all of us.
And given what Bible says -- like, no US in prophecy, at least not directly -- at least we might be the one going down.