I would imagine that is because the highest margins are made off of next-day deliveries from business to business. They often guarantee these deliveries by 10 AM because many companies are willing to pay extra to ensure that their package is delivered by 10 AM.

Consumers, by contrast, are more interested in paying as little as possible for shipping. Shifting their delivery hours to accommodate a group of customers who aren't willing to pay extra for the service while simultaneously killing off business from their most profitable customers would likely be a poor business decision.

Maybe they feel that even people doing delivery jobs are entitled to be home with their family at the end of the day?

They would have to split the fleet, all business deliveries still need to be made 9-5.

Amazon have actually started doing this in reverse in the UK, you can set an address as a business address and your package should only be delivered 9-5. In practice the 'Amazon Logistics' drivers (the aforementioned blokes in cars) are terrible and rarely manage to get this right.

Before I graduated high school, I lived in rural-ish area where the nearest commercial couriers were 50+ minutes from my house to the north and about 90+ minutes to the south. Wal-Mart was literally my only choice for consumer goods, so I spent a good chunk of change online.

I always had the same driver for a certain courier. I asked him how often he had to come down that far, and if it was significantly out of the way compared to the rest of his route. He told me that there were occasionally other stops in my town, but it wasn't the norm. The city and the immediate suburbs were the vast majority of their deliveries. Driving 100 minutes round trip for a couple deliveries probably sucked.

Apart from that it's a good idea.