I think it's not because of Turkish; as Alex pointed out the English noun "sandwich" needs an article (or the number "one") in singular. I guess you could translate "I bought sandwiches…".
A sandwich is a distinct object and not an uncountable substance such as sugar, flour, or beer. I suppose that "bira aldım" could be translated by "I bought beer", without an article. Sorry, English is not my native language (as you will have guessed).