I saw the following sentence in this article. Hong Kong politician hired taxi to fetch TOILET PAPER | Daily Mail Online
Many have questioned why she couldn't have gone to a local convenience store instead of travelling to her former home in the exclusive Peak district.
I am confused about the usage of couldn't have gone. I learnt from this thread that could could be an epistemic modal, which refers to possibility. But I do not think the author is saying it was impossible for her to have gone to a local convenience store at that night. So, how should I interpret couldn't have gone here?