I think "few days" is also a sort of baked in phrase from the early days of large email lists.

I managed a multi-million member mailing list in the late 90s. It would take 24+ hours to send a single mailing, despite us writing our own multi-threaded SMTP senders, per-destination optimized number of concurrent connections (too many and you get blocked, too few and it's too slow), highly managed hard/soft bounces to keep the list clean, etc

Aside from the "day to deliver", it was still pretty common for mail providers to have deep SMTP queues that were very far away from real time. They would very commonly hold mail in batches and deliver them a couple of times a day.

So, we would say "a couple of days to unsubscribe" to account for all that. I noticed other large email lists using the same terminology.

Then, the smaller lists would sort of "cargo cult" this messaging because the lower expectations made their life easier.

My guess is that everyone held onto that as email delivery tech improved to give themselves more leeway, or sneak in a few extra "last" emails, etc.