Power is voltage times current. If you supply 12vDC instead of 120vAC (1/10 the voltage), you need to supply 10x the current for the same amount of power.

To supply 10x the current, you need substantially larger wires. Wires are heavy. Airlines don't like heavy, it costs a lot of money over the lifetime of an aircraft to haul non-fare paying weight around.

Another point, 12vDC is not a typical aircraft voltage, so a DC-DC converter would be needed to generate the 12vDC. 12vDC is typically not a native laptop input voltage (19vDC is more typical), so another DC-DC converter would be needed. At that point, an inverter (DC-AC) and normal laptop power supply (AC-DC switcher) makes more sense (efficiencies are similar).