The main problem is that up in the high areas, there isn't much to live on.
However, the Scottish hills aren't that high really. Retreating up some hills would be a problem, as you could be surrounded until you came down. You'd really need to retreat into a range rather than a lone hill.
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The history books would have you believe that nobody ever climbed a mountain before the 19th Century but that is presumably nonsense?
Some of them were presumably climbed for religious purposes in the distant past, but you'd have to ask why anyone would want to go to the top of some of them.