To stop people using it. The network wouldn't be able to cope if it was cheap so they price it to limit use. Competition has made it cheaper than it was,but it is still very expensive.
In general, these things are priced to limit use. With landing telephone calls the cost is mostly in the infrastructure. A call really costs nothing. Again, they are priced to limit usage.
Keithstructure. A call really costs nothing. Again, they are priced to limit usage.
Your wire/fiber broadband infrastructure has been paid for
many times over and is still digging out from overcapacity.
Your wireless requires them to add capacity now and
ongoing. More channels, more bandwidth, more towers in
NIMBYville, etc., etc. They are in it for the money and even
where price is regulated they will get paid.
Plus they know early adopters have more money than
sense, and price to suit. ;p