We don't use the subjunctive very much in English any more. It is the form of "were" above, or for instance, in "if he were rich." A present subjunctive is often "be," as in "I insist that she be here." Some languages have a full set of subjunctive forms, which is a huge pain for an English-speaker and must be even harder for an Indonesian-speaker.
I understand what you're saying in that it would make the English meaning a little less vague and give it more specificity that the Indonesian sentence does not have. But I have come across other questions where both "leave" and "go" are accepted, since they can be synonymous in different contexts, so there should be more consistency with this on Duolingo.
In fact, I even just tried "Jika Anda pergi, saya pun pergi." in Google Translate out of curiosity, and Google translates it to "If you leave, I leave." If you reverse it, Google translates that English to "Jika kamu pergi, aku pergi".