it's just stolen from other working class people

True, but I'm not the one who stole it from them and no amount of investing will make it so that I am. It was already stolen, and nothing I have the power to do will return it to its rightful owners. However, any value I can extract from it is a little more in the hands of the working class that it was stolen from and a little less in the hands of the ruling class who stole it. That is the least harmful thing I have the power to do with regard to the value tied up in the stock market.