I’ve left London now but did love it there for a while.
I grew up there so family/friends/connections are a big reason people live there and stay. It’s shit that people are being forced out of their home town due to the housing crisis.
Diversity - it’s a diverse city, so that has its own appeal and a lot of people feel more at home in London due to the opportunities to connect with others through community centres, campaign groups, charities, language groups, different faith groups, multi-cultural supermarkets, restaurants etc etc etc. Obviously other cities are also diverse and have their own appeal but London really has it all. So if you’re not British-born, or you’re LGBQT+, or you’re Muslim - it’s easier to find people to relate to in a big city like London. It can be alienating if you feel different to others and there’s nobody else around ‘like you’.
It’s a major city with all the restaurants, bars, galleries, theatres etc you could want. There’s a buzz. You will never run out of things to do. That’s a big part of ‘standard of living’ (somewhere like NE Wales wouldn’t appeal to me - no offence at all intended @Waahaawoowoo but it’s just that different people want different things from life).
It is just a beautiful city (although it does have some ugly bits!) As a Londoner wandering around town, past St Paul’s or along the South Bank or up on Parliament Hill it is gorgeous. It’s fun, lively, amazing architecture. It has lots of distinct areas with their own personality and lots of choice.
Work. Lots of jobs you wouldn’t really get in other cities, or not to the same level. I think this is gradually changing with more remote working and more roles moving outside of the capital, but there is more work and better work for many people in London.
Loads of other stuff like logistics of being in a major city - for example I used to go away on holiday all the time from London as there’s a great choice of flights, from where I live now there’s less flights and so I travel less.