Hard for me to explain why, without being outing, but I have worked with many trainee solicitors at corporate firms. I regularly interact with senior lawyers at them too and am friends with several (from attending law school together.)
It depends a bit on what one considers “worth”. NQ solicitors in those firms have fees targets which are approx 3 times their salary - something that can be achieved given the high rate at which their work is billed to their corporate clients. Those firms are ruthlessly profit driven. They will be keeping an eye on the future profitability of those solicitors - so even if they aren’t making huge amounts of money for the firm to start, the firm will be willing to pay high salaries for the profit they can make off them later as a way to attract and retain them.
The work at such firms tends to be done in teams on one or two large cases at most and there is a lot of boring but chargeable admin work which has to be done by someone and usually falls to trainees and NQs. They are pushed to do it for long hours to meet their targets.
At the junior end the solicitors don’t do what I consider “law” as there is very little advising, very little significant drafting, very little thinking through cases etc. It’s basically well paid donkey work and frankly, it may make them a lot of money but I would find it boring.
Ignore the posters on here who seem to think that such lawyers are cleverer than local solicitors. They’re often not. Being very clever doesn’t mean you will go to those firms - not everyone is motivated by cash. And those outside the profession know nothing about the hours of legal aid lawyers so ignore them too. We often work similar hours but for less financial reward.
I do something very particular and am well known (in my field) for what I do - so much so that I have been head-hunted by bigger firms who act for the defendants. I’d hate it. Like you, I do a job that serves a particular client group. I make a huge difference to individual lives, helping people who need help but can’t afford it. The public law cases I do have wider impact too, which is worth more than money to me.
If you want to switch to corporate law you probably can. But why would you want to?!