It was normalized back when the first video rental stores opened decades ago. It remains today because there needs to be some way to differentiate between a rental and a purchase, otherwise everything would become a purchase at a significantly higher price point.
Maybe the limit could be 72 or 96 hours instead. Or you could rent it with no time limit but maybe can't ever rewind then you can make it last as long as you need but when it's done, it's done.
That said, I don't know why the 48 hour limit on rentals got normalized. I've fallen asleep, gotten distracted, etc. while watching a movie and I don't like now being forced to watch it soon.