because it needs more space

Firstly, for the same power transfer, lower voltage results in higher current ! Thus, the wiring conductor cross section has to be large so that at the high current, very small voltage drop takes place due to load. Remember that 10% drop in a 12volt system is only 1.2volts ! Thus, cost of wiring goes up.

Secondly, at the high switching frequency of the converter, the wiring reactance will become high, restricting switching frequency current components and hence distorting the receiving end voltage with added high voltage spikes (due to stored energy in wiring inductance) ! Thus the capacitor at the DC input of the connected equipment has to be increased in order to cater to DC supply from a long distance.

Check this type of outlet based on 110 VAC supply:

http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=T5632-W§ion=63676&minisite=10251

You will meet the standards about wiring, outlets and products if you use one of these.

The only extra advice is if you use only USB outlets, because you need to justify in your  demand calculations why those power outlets may have low VA load.

I think the short answer is that 12 VDC might have become a fairly popular standard for hand held appliances these days, but it has not been historically. And having to distribute both AC and DC means you have to duplicate that house wiring.

Like you say, there are many appliances that work just fine on AC. A stove or oven, toaster, water heater, incandescent lighting, anything with an electric motor (including clocks, which use the AC to keep time), all of these appliances do not need to depend on any central AC-DC converter. They use AC "as is" just fine.

But many electronic appliances need different DC voltages. Certainly, my audio amps do. So do my clock radios. So it is easier to provide AC to such appliances, and then let the appliance transform that AC to whatever other AC or DC levels it needs.

If you do supply DC to wall sockets, for use in electronic appliances, often you'll have to use switching power supplies in the appliances, to convert that DC to AC, transform to whatever voltages you need, then back to DC again. Which only results in more hardware and more EMI problems.

In short, the fact that handheld appliances these days seem to have converged on 12 VDC for power input is more or less something new. I have transformer wall plugs that operate at any number of other voltages, including 4.5 V, 6 V, and 9 V, for instance.

It is an interesting proposal.To the above comments I would add a safety aspect .One must consider users who have limited knowledge of electrical outlets although in Hotels etc we do find charge stations as mentioned.Standardization/energy harvesting / contactless  charging are other aspects worthy of consideration. Carrying forward the proposed idea a standardized universal dc charger module to plug into the different ac pin outlets across the world would be a good business / useful idea.

Cheers

In telecom 48V has been used a lot, but they are gradually moving to 300Vdc batteries, which is a floating + and - 150V, high impedance earthed. They use a local converter to get 48V. The 300Vdc has lower losses than 230Vac and has the same safety risk, as DC is not triggering muscles, and has a lower risk if 300V is a high impedance +- 150V. I have my own rule of the thumb, the one ohm rule: I like "more volts than amps". This means that 12V would be good up to 144W, 300V up to 90kW. 48V up to some 2.4kW. It explains a lot the used voltages in vehicles. The 12V in a car is even awful: the generator is 14.0 and the lamps are operating at 13.0V. The diesel glow plugs even 10 or 11V. The wiring has an awful efficiency....

I have still to admit that 12V is good for LEDs, they are often only 3-5W. But for a whole house 24V would be better. Another problem would be that a large rectifier may have a larger no-load loss, and not be energy efficient. I use a 12V locally in an atelier where light is powered from wind. The ppt is open access.

https://biblio.ugent.be/publication/4227733

The problem is that not so good plugs are available. The 12V car plug is not reliable, and 115 and 130V should not be used to avoid mix-up (in contradiction to the suggestion above). Another remark is that BLDC fans on 12, 24, 48V are much more efficient than 115 or 230V ones, which are single phase induction motors. 4W DC =16W AC...

for practical experiments we can use the low power rectifier.for home.industrial application we are using high power tranmission line contains a high voltage transformer rating so we must use high power AC-DC rectifier instead of this we are in advanced technology in a low voltage VOLTAGE REGULATORS are used.it can maintain the voltage as low and increase the current level.a best example is DC-DC CONVERTER in the CPU motherboard

In general, using a number of small units instead of one big unit increases reliability. In case of failure of big unit entire system collapses. In case of small units only load of that small unit suffers.  

Thanks for your helpful answers.

because it needs more space