We do use those functions, but we still need to read the file in various cases. We could look to move the file to a temporary location, but that's likely a waste of time in 99.9% of cases (especially if the temp folder is on a different partition and the data has to be copied over) and creates greater possibility for orphaned temporary data.
If your upload temp directory is outside your open_basedir, it's usually a best practice in general to move it (or to expand open_basedir to allow access to the temp dir).