The why is simple: Power/frequency limits allow everybody to be able to use personal consumer devices without licensing and without preventing other people from using
their devices. If you modified your WiFi to blast out a 10W signal across multiple channels at your house, you completely ruin my ability to use Wifi at my house next door. Radio spectrum is shared by everybody.
As for reading there's CFR title 47 [0]. Parts 15 and 18 are germane for unlicensed radios and electronic devices. Parts 22 and 24 cover cellular devices.
The regulations don't explicitly say anything about firmware but to build devices that follow the regulations end user modifiable firmware is an implicit restriction. Even user serviceable antennas are restricted because radio device licenses cover not just the electrical output but total gain of the shipped antenna.
[0] https://www.ecfr.gov/current/title-47