The reason 6Ghz was introduced with WiFi 6E and 7 was because 2.4Ghz and 5Ghz was very busy.
My question is why isn’t there anything in between? Why isn’t there a 3Ghz, 3.5Ghz, 4Ghz, etc?
Also, what if things that require very little data transmission used something lower than 2.4Ghz for longer range? (1Ghz or something?)
I’m not sure what exactly you mean by “in between” here (between 2.4 and 5 GHz?), but commercial radio in the US is at much lower frequencies than wifi bands. AM radio is typically measured in KHz (the range is something like 600-1400, I can’t be bothered to look it up), while FM radio is in MHz (around 87-108 MHz).