There is nothing in Part 90 that prevents a multi-band device, Motorola, Harris and Kenwood both sell multiband P25 devices (V/U, U/800, V/LTE, et al). Motorola, Harris and Kenwood (tait as well) also support multiple trunking/signaling systems on the same device as well.

What you're running into is agencies not wanting to interop - GMRS is a special case, because its a licensed by rule service (Part 95), but everything else, everything public safety and commercial can all be loaded in one radio, legally and technically (GMRS will technically work, its just not strictly legal - but the rules for Part 95 are widely flouted) - it takes agencies buying multiband radios (they're expensive) - and agencies willing to cooperate to share system keys (and encryption keys) which they loathe to do, because it means giving up control.

Interoperability is a huge issue with public safety, and it wont be fixed by broadband, it will only be fixed when a bunch of people die directly because of it, and then the public demand something be done - these agencies have no incentive to cooperate (quite the opposite, they might get less funding if they did).