To be blunt each hire bike I've seen there has been either shite or dangerous.
you've obviously not seen any of ours for the past few years..
I'm all for people bringing their own bikes – I certainly would, and did before I moved here – but there are many reasons someone would hire a bike when they're here: They can't be bothered to bring theirs and would rather not have to clean / maintain it / fix it when they get back etc. Or they simply can't bring it for whatever reason. or they want to ride a different bike – perhaps more travel, bigger brakes etc. And don't want to jack up / weigh down their UK bike by alpifying it.
Like I said – I'd bring my own, but some people prefer not to. Tarnishing all hire bikes as shite, dangerous, or naff is plain hire bikeist 😉