In the same vein as Reddit's ELI5 (Explain Like I'm 5) For Us Laymen (FUL) on this topic, why can't Apple/Google currently default to a Full dropdown to 2G and give the option for by choice users to never allow dropdown to 2G (or 3G/4G) at the OS Level?

Is there a legal mandate (911 Surface Area increase), a hardware impasse (phones physically can't) or is it a company decision to not allow (for whatever reason)?