Expected life of a hire bike? Can't be more than a year or two. I expect the bigger hire companies want to sell them on before they need to start mucking about with anything that's more complicated than replacing a gear cable.
And then the expected duty cycle - how much of the time is it sitting idle? Somewhere like Rutland Cycling at Witwell is going to see a very different pattern to a city centre based multi-day hire. I expect the trail-oriented hire demand is very peaky around warm summer weekends, with a lot of the fleet sitting idle the rest of the time. You also need to stock a range of sizes (if there isn't a bike that fits dad, the whole family will go elsewhere), with the less popular ones seeing much less use.
I suppose a lot of the cost isn't the bike itself. You need somewhere to store them, and staff. Which is probably easier to manage with a smaller fleet operated as a sideline to some other business (bicycle shop, hotel, etc.) than a primarily hire business with very seasonal demand.
The cost of weekly hire always starts to make buying a cheap bike seem like an attractive option, if you can accept the faff of finding a second-hand bargain, or the spec of a low-end bike from Decathlon or similar.