[shagie]

Malicious transmitters are illegal. There is liability for the person operating the malicious transmitter along with the sale, marketing, and manufacturing of the transmitter.

If the maker of a phone allowed a user to break the law by having the phone become a malicious transmitter and the phone maker didn't try everything in their capacity to prevent it, they'd be in trouble too.

Yes, you can hack your own. You can get a CB radio and boost its power by replacing parts of it. That's on you. If you were able to get a phone from a company that knowingly allowed you to install some software or do this "one silly trick" that allowed the phone to broadcast at 10x the power, you'd be in trouble - but so would the company that made the phone.

[stetrain]

Sure, but we already have consumer devices with root access that have radio transmitters. Is this a common problem with laptops? Why would it be a larger problem with smartphones?

[shagie]

The amount of integration between the system and the wifi modem, the frequencies that it can broadcast on, and the regulations for that part of the spectrum.

You'd be quite challenged to make your wifi modem connect to an access point a few miles away. Phones do it all the time. A phone may be broadcasting with as low as a milliwatt when near a transmitter to a few watts when further away ( https://en.wikipedia.org/wiki/Wireless_device_radiation_and_... ). Wifi has a much smaller range of acceptable broadcast power available (and at the most powerful end of acceptable is less than a phone).