I suspect that people who live in other parts of the country just don't see how much money gets eaten up by living in London.
I moved to London from a midlands city 8 years ago for a 10k pay rise which I thought would be great.
My rent went up from £450 pcm for a two bed flat, on my own, walking distance to work to £730 for a room in a shared flat in zone 2 (there are arguments for living further out but money saved on rent is pretty much eaten up by higher commuting costs). There was no way I could afford to live within walking distance of work. I sold my car which saved about £150 a month but spent the same on a railcard, higher council tax etc. Cost of food was much higher as had limited choice of small supermarkets in walking distance rather than being able to drive to a superstore.
So having gone from a wage of £24k to £34k, which actually only increased take home pay by about £400pcm once increased tax, student loan payments etc are taken into account, I found that all of that pay rise was eaten up with increased living/commuting costs and I was in a much worse financial position than I was before.
The thing is it has probably paid off as my friends who stayed behind have struggled to get promoted much above the 30k level as there just aren't the opportunities whereas I have jumped positions a couple of times and am now on 50k.
The relative positions are difficult to judge though. My old work colleagues in the midlands all own properties in decent parts of town either in couples or alone. No one can afford private school and I know they struggle with childcare costs etc but they live in nice houses with gardens in good parts of town. Or if they are single without kids they live in nice city centre apartments. I couldn't afford to borrow enough alone to get more that a studio/titchy one bed miles from work.
Having a partner has made a huge difference. We have a very good joint income of 100k+ we live very well and are incredibly lucky. We still can't afford a 3 bed house with a garden in an area with good schools though which is a bit incredible to me as we are the 2%. It seems mad.
Of course we are still immensely privileged compared to people living and working on nmw, there is absolutely no comparison. But I do think that you probably have to earn 20% more in London to have a similar lifestyle to people living where housing costs are cheaper and I think that skews people's perceptions of what a high salary is.