Same with the Scottish central belt. There are plenty of good jobs up here. Part of the reason for the London centricness is that relatively few people realise just how many good jobs there are outside of London.
All the jobs are a few clicks away from being found. London probably offers quite a few amenities that people want, even though they hate many aspects of it. One of the crucial amenities a big city provides is security that if you lose an income stream, you will be able to get another easily.
If you're a software engineer that security exists in every major city. Plus the cost of living to salary ratio is so much better that it's far easier to save an emergency fund.
