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Famously, loft ladders are cripplingly expensive!
It's more the additional labour costs of folks comfortable working at height, additional time taken to do the job at height and the associated H&S tax.
Most folks are happy just have a working heat producing device, let alone be picky about where it's positioned. 99% of us are gonna get these things slapped in our back yards, in the same way our boilers are slapped against the most convenient externally facing wall so we can run a flue pipe into our neighbours faces.
Servicing them is gonna be expensive enough never mind if its up on the roof.