Old cable guy here. Back in the day people had their modems jacked directly into their PC, so directly on the internet.
LOL, anyone remember needing something like Zone Alarm as a firewall?
TRIGGERED
Maybe it made sense to swap their IPs on reboot for their own, dumb protection? Maybe more likely that now we all have 24/7 modem/router/firewalls it costs less overhead to leave the dynamic IP alone? Maybe they just want us to pay extra for a “static” IP, even though it’s basically that way for many or most of us?
Your last point is probably more on the money. I WFH and have to have my IP in a couple of whitelists. Since it never changes, no hassle.