WHY aren't voice & "3G" cellphone frequencies simply updated with SOFTWARE for switching carriers??

For example, when switching a GSM  device from T-Mobile to AT&T...

Or a CDMA device from Verizon to MetroPCS....

Or, when a carrier buys a new range of frequencies - like MetroPCS did in NYC....

This SHOULD all be a "software update" and/or settings...

Why isn't it?