Edit: To be clear the repayments on a £1m mortgage would "only" be like £60k a year, you only need £>200k to initially buy the place.
1. It's about the money, because you never know when you may have a 5-10 year underproductive period due to health/other unforeseen emergency.
2. It's not about the money, it's about the experience/knowledge gained/distraction/self-worth of accomplishing more.
To me, a side gig means accomplishing more of what someone else wants, with the trade-off of being less able to accomplish (or learn about, or distract yourself with) what you want. Which seems like an odd thing to want unless you need the money. I suppose for some people having a £200k income and not needing the money seem less synonymous than they do to me.
See: John Carmack at FB.
The money outcome isn't a zero-sum game. If someone works more, they can put that towards their children's education fund, retirement or other investments, or anything else at all.