This doesn’t have anything to do with email, but along the same lines…

Why can stores take money out of my bank account immediately when I pay, but when getting a refund, it takes 3-5 business days?!

Hell I've seen 30 days. Sometimes they are using a third-party - it still should be almost immediate these days.

I used to work in online promotions, things like collect and win, enter a code to win an xbox, that type of jazz...

It's not that the local deletion can't take place immediately... It's that they have to legally cover their asses because there's a whole chain of backups, mirrors, scheduled things blah blah... It literally might take that long for all of the changes to propagate through the full system.

Usually once you unsubscribe, you're added to the unsubscribe list, and then the local deletion happens with the nightly Cron job. It really is almost done immediately but they just legally cover their rear ends from that point forward because there is a chance it does linger somewhere within the greater system

I'm not going into all the technicalities but that's the basic reason

I used to do data reporting and list generating that sometimes involved working with advancements marketing. The email marketing system set a static list before it was actually sent. So you could have a draft ready 3 days ahead of time and each unsubscribe wouldn't impact the static list they already set up. They eventually upgraded to a better system, but I could see situations where 3-5 business days is actually necessary, simply due to poorly planned technology.