For me it all boils down to them all being cities big enough for you to be able to do anything you can think of without needing to really plan ahead.

Want to go see a gig that day? Pick a genre, it'll be on somewhere. If it's a big international tour, I can assume it'll be on at a venue near me.

Want to try some obscure cuisine? Enjoy a huge selection of restaurants or food halls to visit serving anything you can think of and several companies who will deliver it to you in half an hour. If you're not feeling adventurous, your favourites will be available from countless places to a very high standard.

Want to walk round some galleries or museums, or even just a tour around somewhere with historical significance? Take your pick

What about sport? You can go and get tickets and watch some of the best athletes in the country or even the world.

Have a niche hobby? There will be a group of people you can share that with somewhere, there may even be several groups.

Want to get from A to B? Go to the metro/tube/tram station, one will be along in less than 5 mins. You're not forced to own a car.

Want a bit of nature, they all have parks with fantastic botanical gardens all over the city and great countryside & little villages just outside of it.

Nightlife? You'll be able to have any kind of night you feel like: a big pub crawl, dancing into the early morning, a laugh at a comedy club, maybe just a chill out at an arcade and everything in-between.

Need some random item delivered? You might be able to get it in a couple of hours, definitely by the next day if not

Want to just experience something new? They're always changing, so you'll never have seen it all

Need to get to another major city in the UK? Rapid trains to get you there. Need to get to another city on the planet? It's not unreasonable to expect the airport 30 mins away to be able to get you there directly.

Finally (though least relevant to you) there's work, you simply get a much greater selection of better job opportunities and easier commutes to those jobs if you live near them.

(that ended up being a touch longer than I expected, haha)

Edit: fixed a couple of things