Because its cheaper to use oil Currently oil is about 60p a litre as domestic fuel. There are about 10kW in a litre of oil, or at say 60% efficiency 6kWh.
That equates to 10p a unit electricity, and if you know where I can buy that, let me know.
Likewise the Aga is probably around 90% efficient at space heating.
And a heatpump even at a conversion efficiency of 300 % is hard put to equal 30p a unit these days.
If the powers that be came to their senses and knocked out reliable nuclear power at 10p a unit, I would probably use the immersion heater. And scrap the oil boiler not for a heat pump, but for a simple electric boiler. It would cost around £20,000 to put in a proper heatpump here, and probably require 3 phase electricity.
I can but a LOT of oil with that, probably enough to 'see me out'
But as long as the game is utterly ruinously expensive unreliable renewable energy, oil is in fact the cheapest option at the moment. And the Aga at least doesn't need electricity to work, it just makes it work better by regulating its temperature automatically.