Because they were already in a stalemate with France, the UK, and Russia. And their allies the Austro-Hungarian Empire were more concerned with Serbia and Italy

Because they were already in a stalemate with France, the UK, and Russia. And their allies the Austro-Hungarian Empire were more concerned with Serbia and Italy than France, etc. So this was a move intended to counter the US should we enter the war ("give them something besides Germany to worry about" was likely the thinking)