They're a toy being sold in small numbers to people who live near lakes or can fit driving to the lake with it into their life easily enough to want to buy it - good proxy for having a bit of disposable income to throw around. Market can bear a high price, not enough volume to make competition appealing => ££££
I spent a bit of time looking into building your own - from a parts point of view (buying new) you are still looking at £1000 to more like £2500 worth of batteries, controllers, motor and prop, plus the foil and the board you attach it all to. Then you need to make, seal it and make it 'sea worthy' - would be hundreds of hours of work for me.
I'd happily buy a well made new one for £3000 if that was what they cost, but half that price is a new good solid paddle board - so I can see why you can't get the same plus all the electronics you need for £3k.
We can hope in time the costs come down, but as a niche sport, it will be some time, if ever. Have you seen the cost (and service intervals!) of a new jet ski? All simple tech, but they don't come cheap and need a lot of looking after.
I, frankly, don’t understand it. They aren’t particularly advanced devices.