Afghanistan is where Empires go to turn power into corpses.

That Russia for probably years has viewed this as a good way to blunt the might of the USA is almost a certainty. And there are probably others, as - why not?

There's a good chance Pakistan (who only gets subsidies as long as there's a problem to correct), China (ensures the nutters go West from their turbulent provinces and keeps the USA's focus away from reef building.

Giving weapons to jihadists is probably the cheapest and least risky way of attacking your enemies. And by doing it in Afghanistan it forces the USA to fight at the end of an extremely long logistics road which only adds to the cost. In Syria there is Israel's extremely efficient - not to mention ruthless - intelligence network which will destroy any weaponry they find and view collateral damage as something that happens. Who will help the Yanks up the mountains? The locals??!?

Equally, none of the other players want America to "win" or "loose" since then they'd leave - and many of the more zealous fighters will start looking for the next battleground. A nice long stalemate where the aid is carefully balanced to what the USA does to ensure lots of time, money and materiel is wasted or at least tied down.