Financially, I don't disagree.
The amusing thing is that I often would say the exact same thing on the other end of the coin. Going to industry right after your undergrad is one of the worst things one can do intellectually. People who want to grow intellectually[1] typically will grow more in their grad school years than most people will in their life. I can clearly see it when I look at my friends who did not opt for grad school.
And even within a particular discipline, for a lot of disciplines, there is virtually no way you can attain that particular knowledge while having a full time job. Life gets busy.
It ultimately comes down to values. Some prefer the money, others the knowledge. Personally, I appreciate the adage: Money comes and goes, but knowledge tends to remain with you.
> On top, they learn no marketable skills that can get them a job out in industry or corporations.
People who plan and aim properly for a career in academia probably pick up more marketable skills than the average industry employee. Self promotion and networking matters more in academia than in industry. There's also stuff like grant writing, public speaking, etc.
[1] Not all people who pursue a PhD want to grow intellectually, but a significant portion do.