There is a running joke in the industry, If someone comes up with 'law of universe' then they would be the richest person in the world because they could predict the stock market.
People have been trying to predict stock market using mathematical models since the dawn of computers, Alogrithmic trading is allowed by most exchanges and Hedge funds using Quantitative models have largely been successful.
So the answer for why there is no AI service which I can pay $5/month to get profitable returns next month is because no AI could predict stocks for short-term movements due to large number of variables.