Primarily because of travel distance.
Electric dissipation to heat through a wire is a function of the voltage (among other factors). This is why powerlines are run at extremely high voltages.
In consumer gear: interconnect are generally quite short; and 1v gear is cheaper to built (There's also likely some historical "how it was already done" elements involved).
This is also why professional interconnects have interference rejection (via an inverted signal path in XLR) and consumer does not. While XLR is, simply put, better (as is 2v operation); it's also more expensive and unnecessary in most home applications.
Close enough, except for the power line explanation (that's due to increasing the voltage and dropping the amperage so that the transmission lines can be efficiently sized... high currents (amperes) require larger gauge wire. Otherwise, resistance rises due to hear and power transfer is limited).
So unless you're running audio cables all over long distances, why require something with 2 volt output or input? There's no reason. In fact, it complicates things because matching components (pro and consumer) can lead to problems (including an increase in noise). So there's no real gain (pun)!
The same thing with XLR connectors. While the do offer real benefits, using them in a mix of gear (pro and consumer) can lead to ground loops. Why? Because pro gear is grounded, most consumer gear uses phase plugs because it is double insulated.
Ground loops can sometimes be resolved through the use of a ground lift switch, but doing this on pro gear increases the shock hazard because an active ground pin is an essential safety feature in gear that is not double insulated.
Anyone with a rudimentary appreciation of how much power is needed to achieve reference levels knows that you don't need hundreds of watts to drive the vast majority full range speakers to fill the vast majority of home spaces with deafening sound. So if you don't have large distances to cover between components or require balanced connectors, why do you use pro gear?
No need to answer. It's a useful rhetorical question for other readers to ask themselves when shopping for gear.
