Stock investments by retailers may not result in money directly expendable for companies' capital investment expansion, but they help to sustain their market cap, their equity value, and therefore their ability to obtain bank financing or other type of direct institutional investment. Your alternative suggestion, to introduce GST on trades, seems a variant of the so-called Tobin tax. It would probably result in a reduced  level of short-term trading:  would this effect be beneficial to the economy at large and  to equity values, i.e. to the value of your little investment portfolio or mine?