It's because their insurance etc doesn't cover renting out.
Even the solo time requirement, still talking about the UK schools, is for me bit unusual in terms of conditions. The 'deductible' of 5k (IIRC) has to be paid down cash in case student dings/crashes the gyro on the solo flights. I mean, it is what it is and the guys don't hide this info. Even with some sort of 'renter's insurance' UK style, the cash downpayment to finish off the licence requirements, isn't exactly incentive. There are guys who may not have cash ready (or desire) to buy their own gear.
Not sure about you guys, but I don't have spare 5k GBP to put down as security on top of other costs. Other than that, I may have done some gyro flying in the UK. I don't get to do it in the US where I fly heli or fixed wing.
Another thing, with gyros, is that very few models have UK CAA's approval for training. So there are two seaters that can't be used for training in flight school. What the owners get themselves to fly privately in experimental/PtF regime, is their business.