Hello @SophieWong - if uploading multiple files to an assignment is a desirable feature, you are able to submit your suggestions here to the ideas page.
https://community.canvaslms.com/t5/Canvas-Ideas-and-Themes/ct-p/canvas-ideas-themes?tab=identified
But if it were me, I might also look into a different tool to maintain my files. For example, a cloud service that enables me to store my files online but also have a synced local copy on my personal computer. Such tool options are many and include Microsoft's OneDrive. With such a tool you can then drag and drop multiple files from your local file explorer into a Canvas assignment file upload interface.
Also, just to confirm we are aware, when we upload files to a course assignment a copy of the file is placed within our Canvas Files > My Files > Submissions > Course name folder.