This is more of an excel issue. If you open the csv in notepad the leading 0s should be there. Take a look at the below link, essentially import the csv into excel via the data ribbon instead of double clicking the file and opening it in excel. You have much more control by importing.
https://provider.www.upenn.edu/computing/da/bo/webi/qna/iv_csvLeadingZeros.html
@Luke_C ahh, yes, you're absolutely right. i should have thought to check that first. thank you