One large part of this is simply lack of education.
The other part is the realization that if making close to minimum wage, investing in an index fund would still take quite a very long time to go up significantly.
Give the choice of waiting 40 years for not a huge payoff or gambling your 5K-10K of savings for a potential 10x return in a year or two, it's easy to rationalize the latter.
Of course, now I'm fortunate enough to be a well paid SW. The marginal utility of every dollar above 75K is low in the present, so I can put all that in investment funds.