It could be the 20+ years of using linux where it was standard for it to only have files there deleted on boot. Temp in this case means it's temporary for that boot. Recent redefinitions are surprising unless you've only come to linux recently or from unix 40 years ago.

I am going to assume you are used to tens of days wipings of /tmp. Imagine suddenly it was decided that the wiping needed to happen every tens of seconds for security reasons and to minimize ram usage. Do you think this would impact your workflows that used /tmp? This is 100% analogous to the current situation. Yes, tmp is ephemeral, but on what timescale?