There are less than 4 million miles of road in the US. At 1$ per mile you can send a truck down every one 6 days a week for 1.3 billion a year. (Note, there would be some backtracking but also some roads are avoided.) FedEx's revenue is 45 billion a year so the last mile is not a problem as long as the volume is there.
On top of that if you live in the middle of nowhere they simply don't deliver to you. So, if more people moved to the desert for whatever reason there costs stay more or less the same.
The drivers alone probably cost more than $1/mile, then you have gas and maintenance. FedEx had $45B revenue with $2B net income, not exactly huge margins.
Obviously there is more to package deliver than just the raw cost of getting a package from a local sorting center to the end of a rural driveway.
At the moment, a lot of the cheap rural shipping is being subsidized by the USPS.
On top of that if you live in the middle of nowhere they simply don't deliver to you. So, if more people moved to the desert for whatever reason there costs stay more or less the same.
https://en.m.wikipedia.org/wiki/Transportation_in_the_United...