To answer your question, because we the people allowed it and we continue to allow it by not demanding it be ended or at the very least supporting candidates campaigning on doing something about it.
By people, you mean the Republican voter. We Democrats can’t put pressure on our candidates about these issues because losing means a batshit insane right wing / nazi / Christian nationalist wins…
So we have to pick and chose our battles. I’ve got bigger issues than multi millionaires being allowed to trade stocks.