Because when paying for 1-day shipping, the cost is astronomical. It would be
unacceptable to pay that price and not have it next business day. If they couldn't actually do real one-day shipping, then they would not be able to offer it. It exists only because we have to pay a large fee on every individual delivery. You pay the high cost for a guaranteed service.
Amazon Prime simply cannot scale at the given price point. People who place >= 100 orders a year are paying <= $1 per shipment. The $100/year Prime subscription is not sustainable if people use it. I'm sure they claw back a significant profit from people who buy Prime and then only order 10-20 times a year, but it's possible it doesn't earn back what excessive users are costing them.
Is it a little shady that they offer 2-day shipping with caveats? Yes. They should either make it much more clear that there are no guarantees, or increase the cost of heavy Prime users to be profitable enough to scale it properly.