That's certainly part of it. Since 5G modulations actually only provide 15-20% increased thoroughput compared to 4G almost all of the advertised "increases in s

That's certainly part of it. Since 5G modulations actually only provide 15-20% increased thoroughput compared to 4G almost all of the advertised "increases in speed" for 5G networks have to come from using new spectrum. And the only remaining spectrum, well, sucks. So another part is the complete fiasco that are the mixed up, rarely overlapping, mm-wave 5G band definitions across the world meaning more discrete antennas are needed in each phone. Additionally the requirement for multiple antennas for each of these actual band for the higher frequencies because they have to beamform. And the power requirements for the frequency multipliers/etc and other frontend RF stuff for each set of these.