"Why do MN poster regularly assert that you get a better quality of life outside London"
Putting aside personal preferences such as being closer to nature / the sea / quiet places etc ...
I think what people are saying, perhaps, is that a decent quality of life is more affordable outside London. The cost of housing for example is astronomical. I'm London born and bred but - like the vast majority of my childhood friends, I moved out of the Z2 borough I grew up in because none of us can afford the best part of £1m for the kind of house we want to bring our family up in.
If I were to move back to London, the most we could afford is a 2 bed flat in an undesirable area. Where we are now, we have a 3 bed semi with garden and plenty of space.
Also, there's the extra effort it takes to live in London. It takes longer to get places, parks and pubs are busier, you're having to fight the crowds all the time - unless you can afford to to pay for other options (e.g. black cabs).
Having grown up in London it wasn't until I'd been out of London for some time that began to notice what hard work London can be. Having grown up with it I had developed the competitive streak you need if you ever want a seat on a tube / bus / to get a table in a busy cafe!
But, you know all that can be wearing after a time!
In the small town I live in, you can drive to the park, park there, walk in. Takes 5 minutes, not 20 minutes circling trying to find a space then 10 minutes walk to the park!
Simple things, but it all adds up to being less stressful outside London. More importantly, in the town we live in, we had a choice of excellent schools. We didn't have to mortgage ourselves to the hilt to get into catchment for decent schools.
If I had the money to buy a house in Highgate, and to pay for options that relieve some of the stress of London living then I'd be back in a shot. But with the resources we have, living in a small town definitely gives us a better quality of life.