The corrrect answer is massive profiteering off of suckers.
There’s some engineering expense, that makes real bikes that last years and perform reliably, which makes it more expensive than a Walmart bike, but after that it’s rip off city.
Easiest measure to illustrate this, is the price of motorcycles. You can drop £10k on road bike or mountain bike, and still not really get top of the range. Look up what kind of motorcycle you can get for that money and then make a value judgement .