They are low effort, but I'm not sure why that's relevant. Effort isn't what's valued. Scarce and useful resources and inputs are what is rewarded. A talented dev who works 3 hrs/day can earn more than a less competent dev who works 12 hrs/day, despite expending less effort. Effort is only relevant insofar as it pertains to valuable output.

On index funds, the reason they are profitable to hold is because of the risk premium embedded in its current price, and this risk premium comes about because of the risk aversion of the average investor. As time passes and the world hasn't blown up yet, the price goes up, as it should. It has nothing to do with effort, it is about risk and opportunity cost. It's the same mechanism underlying the yield spread between corporate bonds and government bonds.