It's not only the issue of SoC modems but about the whole integration (RFIC, LNA/PA, RF switches, antennas...). The more bands you integrate, the more difficult it becomes (or with compromises such as worse performance because of wideband antennas...). If I may risk a comparison : you can't have a lense with a huge focal range that would simultaneously be perfectly sharp and have a wide aperture like fixed focals...