Why can current phones act as Wi-Fi routers, but not as mobile broadband (3G, 4G etc.) routers?
While 3G\4G uses different 'commercial' bands, WIFI and Bluetooth uses the 2.4Ghz band which is the only band that is open for private use throughout the world (There are other bands that are open for private use but different countries have different regulations and not all of them authorize all private bands)


It is easy for major RF manufacturers to build communication components that work on a single known and authorized band.
This guarantees that if you do a tethered network you wont accidentally (or intentionally) mess around with the cell-phone provider network.