Noise, crowded, traffic, pollution, lack of wildlife and nature. Some of the parks are not bad, but are basically a lawn with an artificial lake, giving a bit of green breathing space in an otherwise dirty, smelly concrete hell hole. If you want to do a 10 mile walk or a 40+ mile cycle ride largely free of people and traffic, no chance. Masses of light pollution and everything is very expensive.
If someone came and give me a choice of no change on my mortgage, continue living where I am, with my veg garden, the coast a short walk away, woodland, good quiet cycle routes on my doorstep and farmland just the other side of the road, or spend the next 10 years living in central London in a far more expensive place with no option to sell, I'd stay put.
You can't do culture and entertainment every day. I wouldn't want to use overpriced bars and restaurants every day. It is far better (for me) to live my life somewhere nice, quiet and green, then save museums for trips out. Get in, go to where we want to go and get out, back home to sanity.