The exporters use the "temp" folder within the home directory as returned by Confluence's SAL implementation of ApplicationProperties. In a datacenter installation this returns the shared home directory and not the node-local home directory.
Confluence uses a scheduled job to clean up temporary directories, however this job only clears the node-local temp directory.
Therefore the temporary files created during exports are never removed out of the box.
The exporters should use the node-local temp directories to ensure such files get removed eventually.