"Can my favourite software be included in Tails?" in our FAQ:
https://tails.boum.org/support/faq/ > 3. Systemd private directories in /tmp > Why are there so many "systemd-private-" directories in /tmp
I guess it has to do with systemd's PrivateTmp= security feature.
If you're more curious than that, I suggest checking in the systemd
source code.