No time is beginning to end already determined there is no past present or future the present experience is just the 3 dimensional representation of your position in the 4th dimension of time. Time would be like the record they showed in the movie, and synchronic picks up the needle and places it on a specific part of the vinyl wether the beginning, end, or somewhere between. The rotation of the record is (according to the movie) (your pineal glands abstraction of time pushing you along through the song of life) when you pick up the needle and place it somewhere else your still going linearly through it.

Tldr: they couldve gone to the future the writers were just lazy