Because of this:
http://www.faa.gov/atpubs/AIM/Chap4/aim0401.html#4-1-14
Once you're with them they don't expect you to "just leave".


They've forgotten about you. If they knew you were still with them they'd
have either terminated radar services or transferred communications before
you were so far inside approach control airspace.

I've switched frequencies away from Chicago Center, when I could no longer
receive them (due to low altitudes) -- and they responded by calling the
airport manager at home, and having the poor schmuck go out to the airport
we landed at, looking for us.

They really don't like to have pilots leave their freq without permission.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"