It's fairly easy to handle dual voltage for switch-mode power supplies like used on electronic devices, but a resistive heating element like used in a pressure

It's fairly easy to handle dual voltage for switch-mode power supplies like used on electronic devices, but a resistive heating element like used in a pressure cooker typically runs directly from line voltage, and needs to have its resistance tuned to draw the right amount of current for the power line voltage. A resistor tuned for 120V would draw 2x as much current (and therefore 4x as much power) on 240V, and probably burn out. You can get clever by splitting your resistor in half, and then using relays to put those halves either in series for 240V or parallel for 120V, but that adds cost and complexity. Since most consumers don't need this functionality, it's easier to just sell two different SKUs with different heating elements. High-power motors like used in blenders are similar - you'll use motors with different number of windings at different line voltages, otherwise your motor would be horribly underpowered at 120V or burn out at 240V. Power transformers exist which can convert between 120V and 240V, but they're not cheap.