You’d think electric blankets would use a 12V DC adapter to step down the 240V AC to 12V DC. It has the potential to be just as effective while also being safer. Do you think it’s because of the additional cost of the wall adapter?
Converting 240Vac to 12Vdc involves a lot of subsystems. You need an EMI filter, rectifier, and then a step down switching converter (buck, or H-bridge, etc.). Electric blankets just produce heat and you can cost-effectively due that by just putting the incoming AC on a resistive load and avoid power conversion completely and greatly reducing the cost.