it is exactly this. I am self-employed and have been managing my solo 401k for 15+ years now. while I am fully aware of market being overvalued I will still max out my 401k this year ($80k, I am over 50 :) ) and that money needs to go somewhere. I could theoretically sell and sit on the cash and wait on "crash" but I could have technically done that a year ago (this were similarly not very rosy) or even earlier and that obviously would have been a terrible financial decision. no one can time the market and therein lies the core issue, money will always be flowing in...