I don't think anyone is saying that you can't earn good money outside of a physical job.
What people tend to say is you need to put in a lot of work and effort to expect high earnings, which is good advice.
Don't fall into the trap of chasing big returns with little to no work neglecting working on your skills. Even if you're successful and end up with a fortune, you'd be looking at your peers who mastered their professions, while you'd have nothing to show for yourself other than money. That's not the best place to be.