Not sure about the UK, but in the US consulting has a lifecycle. Very few hires stick it out for more than a few years. But that’s a feature not a bug. It’s not unusual for the Big 3 to help find you a job at a client. And why not? Now they have a lead for new work at the client. And the anti-disparagement makes sure when you’re talking to coworkers at your new job you talk about how great your time was and pass any interested candidates back to your old firm as new hires. Again, now the consulting firm has another contact at the client to help drum up business.