Okay so I've seen quite a few of these why MC/SC posts lately, as a trainee at one of these firms I therefore thought I would put down some reasons why people want to or do work in them. This isn't me saying that these firms are better than others or more fulfilling than other types of law, just saying some reasons why people like it.
The money - people keep saying 'apart from the money' as if it isn't one of the most important factors. But the reality is that even as a trainee I am in the top 1% of earners in my age bracket, I generally make over double what my close friends make. Once you hit NQ you are on 6 figures and earning more than what most could hope to make in their lifetime, usually by mid 20s, and if your interested in partnership could be earning millions by mid forties, so yes the money is a big part.
The work. You're working on really high calibre deals and cases often worth 100s of millions or billions. These deals are complex, interesting and your working with household names like Google, Meta etc. There is also a large variety of work, a lot of smaller firms are more specialised, at top firms you can do top disputes and top corporate work for example.
The people. You're working with some of the top people in the industry. Also people are generally really good at their job and care a lot. It's a real benefit some people take for granted. No one is lazy and your never having redo bad work. You don't get that in every company.
The prestige and career prospects. Yes a lot of people want it for the prestige. If you say you work at one of these firms (MC especially) people do look at you differently, it's kind of like why people want to go to Oxbridge. Also the career prospects, people in these posts seem to assume that people want to work at MCs forever, but it is a lot easier to go down then to work your way up. Someone at a high street firm would find it almost impossible to work up to an MC, but an MC trainee could easier move to high street.
The perks. You get a lot of perks at these firms, eg free gyms/dinners. Free tickets to events. Free or discounted trips abroad, sailing/ski trips. They often have committees and societies that are fun to be on. Not to mention other perks like private medical/dental, travel insurance etc. There's also a lot of free corporate events at places like the gherkin etc.
It's enjoyable. Some people seem to think that working at these firms is just this horrible experience that everyone hates. Yes there are bad days/periods, but day to day I really enjoy my work. The works interesting, the hours for a lot of trainees aren't as bad as people think (I'm usually 10-6) and being with such a large group of trainees who are basically friends is like being at uni again. You are also get actually quite a lot of freedom to manage your own schedule/day.
So yes, obviously there are lots of negatives for these firms (and if anyone wants a list of those just ask!) but there are also loads of positives, and I generally enjoy it a lot, despite some tough parts (but that's what keeps it exciting and helps you grow as a lawyer in my opinion).