Investing for retirement shifted from a company taking responsibility to employees taking responsibility with the advent of 401k and similar plans instead of defined benefit pensions. Over time it has been noted that picking individual investments tends to do worse than simply buy everything.
As workers pour money into these non-discriminating investments, hoards of people are blindly investing and skewing away from efficient markets. Everyone (scare quotes everyone) is just buying large swaths of everything all the time without regard to fundamentals.
It's all good as long as this continues, but a shift in enough people no longer believing this, will reduce demand and prices will go back down.
In other words, people just buy everything because there is no alternative. If enough stop doing that, prices will readjust downward because too many stocks are overvalued due to the current wisdom of buy regardless of price.