First counter-question: why should an individual investor care?
If I had $1 billion, even microscopic improvements in the yield of my investment would dominate any income I could hope to earn through wages. If I had $1, my investment yields wouldn't matter -- doubling my principle would be less remunerative than working a minimum wage job for 15 minutes. Somewhere in between owning $1 and $1 billion there is a net worth where it becomes worth it to switch from a generic roughly age appropriate portfolio to something more finely tuned. I had estimated that cutoff to be many times a person's annual income. If the cutoff is lower than my estimate, then a lot more people should be learning modern portfolio theory.
You're forgetting about the utility function. If you had a billion dollars, there's no reason for you to care about the return of your investments at all, much less about microscopic improvements. On the other hand, if you're retired and you're living on the income from, say, a $100,000 portfolio, any additional percent that you can eke out is meaningful to you.
The real question is why do you consider the market portfolio (which, again, most of US residents understand as an S&P500-based index) to be the default?
For historical context, it certainly wasn't the default for saving money for retirement, say, 50 years ago.
P/S/A: There are single sentences which can create life-changing amounts of difference.