Because out here in the hinterlands mail carriers don’t like driving up 100 foot (1/4 mile) (1/2 mile) long driveways.
And letter slots are hell on heating bills when it regularly gets to -20C.
AFAIK when I moved to my new house where there is a rural mail carrier, there was a regulation that there must be a mailbox placed at a certain height and a certain distance from the street.
They will not deliver to a mail slot in the door.
I live in a semi rural area; it used to be farmland but is now being populated with quarter million dollar houses that butt up to or have a view of the golf course. All the houses sit far back from the road including mine. My driveway is 300 feet at least.
When we first moved in my hubby and son took great pains putting up the box. Dug the hole, made sure it was level, numbers and letters placed meticulously. Now as long as it is upright and the door opens, it passes.
Our box has been taken down at least 10 times including twice by the snowplow and once by a water hauling truck. At least ODOT (Ohio Department of Transportation) will replace or repair the box/post if they get plowed.
The privacy is nice but it is a pain to walk down and get the mail and newspaper when the weather is bad.