Because one guy is delivering a huge number of packages, and the service wants this to happen in the most efficient i.e. cheapest way possible. They do their best to come up with an optimal route for all the packages for the day. UPS famously goes so far as to avoid left turns whenever possible when planning their routes for the day, because it takes up significantly more time.

If you're picking a more specific delivery window then you're throwing off that planning and it likely requires the delivery driver to take a less efficient route than they otherwise would. That costs the company, and therefore it costs you.