One guess is that maybe she had previously had someone she got along with really well (friend or collaborator) but he had caught feelings and asked her out and she rejected him because she wasn’t into him like that only for him to also withdraw from the platonic relationship. So, given that experience, she knew it was a lose-lose situation when you asked her out but decided to try accepting the date but just wasn’t into it at all.

Though another possibility is that she’s had bad experiences rejecting guys (or maybe with stalking) and instead goes with the strategy of accepting the date but making them no longer want her by basically grey rocking them during the date.