Cause its so convenient, fast and efficient. I have kids so I can make food quickly (sous vide'd chicken I've done some days earlier, usually) and it doesnt draw electricity as much as my normal oven. I quite like it, and it makes my life easier.
I orginally bought it like 6 years ago to make chicken wings, but nowadays I use it for pretty much everything.
Because most air fryers are countertop units that plug into a 15 amp outlet, vs most residential electric ovens that require 2x 240v 30 amp connections, or roughly 8x the available power...
That’s a bit of a stretch since most residential ovens also have a range on top, so the power requirements are to cover multiple elements, not the oven alone. It’s probably actually less efficient, but still lower energy because you’re heating a smaller cavity.
That's 14kW a whole range doesn't pull that much on full blast. There's plenty of ovens in the 3600W range meaning they run off bog-standard 240V@16A lines here but chances are you're in luck and your kitchen has 20A three-phase... also 14kW (in star configuration), intended precisely for the range, roughly divided into two for the top and one for the oven. Not much more copper so it became standard quite a while back, while 30A would be massive. Also more neutral wires.
Can't speak for them, but I've had a smart monitor which shows live consumption. Took note of the consumption while using the oven against baseline consumption, and the same for the air fryer.
Air fryer consumed approximately half the electricity for an equivalent amount of time in my case, but it's made better by the air fryer needing less time to reach temperature and cook whatever it is I'm making.
I can monitor my power usage via my solar panels and powerwall. The app lets you see how much you're using at any given time, so I just turned on one appliance at a time to test their power consumption.