[jimmies]

Perhaps because it's not easy to do so?

Now you have to have a database of a million phones from Shenzhen that you need to keep track of whether it has your bands. Who's gonna pay for their QA and know what phone manufacturers are doing what with their phones?

[Laforet]

Indeed, this has been an issue since the 3G era. I live in New Zealand. 10 years ago one of the largest local carriers switched from CDMA to WCDMA-only using the exotic North American 850MHz band whereas all other carriers use the more common 900MHz. They also both have 2100MHz base stations in busier urban areas.

This caused endless headaches for users and tech support alike because the market was flooded with phones that might have 850MHz support. There is no way to tell from the model or serial number - even Samsung and Nokia often failed to properly indicate that in their specs. And because both networks were also on 2100MHz, a phone may appear to work fine for months, but you will find out the hard way when reception completely disappears in a slightly remote area. There was also no GSM to fall back to since they built it from the ground up with no 2G layer.

Legend has it that one intrepid phone reseller would routinely take his stock on a boat to a spot 15km away from the nearest shore where no 2100MHz signal could reach, and only there could one be sure that the reception you are getting is on the 850MHz band.

[dataflow]

Not really. Nobody's swearing under penalty of perjury here, 100.000% accuracy is very much not a requirement. I think a best-effort list would go an insanely long way, and carriers already have large IMEI->model mappings for a lot of phones all over the world. And there are already gazillions of random websites with databases of model->band mappings out there too. And in the absolute worst case where they see a device they have zero clue about they could at least give a slightly different message like "we don't know if this is supported, but please be aware [...]" or whatever, and annotate their profile for the rep accordingly. Again: perfection is not required here.

[Causality1]

A cell tower can't interrogate a handset for a list of supported bands?