Because there too many mobile phones, all built to very different standards, that it would be impossible to test them all against every type and configuration of electronics suite in every model of plane. In all likelihood they are safe, but no one wants to take the risk. So we have airplane mode.

The article specifies that 5G is the band approved in the planes, likely because European electronics standards (in planes) are higher than American, so knowing the frequencies it would be easier to have that one band instead of the 2G/3G/4G range.