mac - Unable to open file because startup disk is full

I was trying to open my excel file from my disk-based thumb drive when i got the following error message

I proceeded to make space on my hard drive by emptying my recycle bin and deleting some stuff permanantly before i could open my excel file from my disk based thumb drive.

Why do i have to make space on my startup drive before i could open my excel file , I thought the file would be temporary stored in my RAM and executed from there . The file size is at max 30KB , I am sure it can be stored on RAM