Updated: Oct 16, 2020
When an investor buys stock with a large sum of money, it can actually push the price of the stock up. This is especially true if an investor is buying into the stock with millions or even billions of dollars. Most trades made by institutions are so large that they often push the price of the stock up when they buy, and they also push the stock price down when they sell.
Each trade, buy and sell, moves the price of the stock. This movement presents an opportunity for smaller investors, usually people with less than $1 million, to follow the trends of the market and make quick and precise trades. Long term investors hold for one year or longer, so they don't necessarily care about the exact price they purchase at, so long as they don't eat too much into their prospective profits. But day traders need to be extremely precise about the price they pay per share, because they're often looking to make small quick gains, so even one additional cent can be detrimental to their returns.
Day traders that are looking to follow these trends set by long term investors are simply filling a niche. Day trading is still small-time investing; it's incredibly difficult to do with more than a few million dollars because the more shares an investor buys or sells at one time, the more they influence the price. So with larger portfolios of money they'd struggle to find enough viable openings in the market to generate good enough returns.
There aren't any day trading billionaires. It is a good way to make exceptional returns with small amounts of money, again no more than $1 million. But a common dream of most people is to escape their 9-5 job with $500,000 in their portfolio and day trade for gains of just 1% a day. $5,000 per day is roughly $1.26 million yearly and you'll never touch the principal. This dream is entirely possible, I know several people who do it currently. But day trading is risky and I don't recommend it. I admire anybody willing to try, but like all other skills it requires lots of studying and practice to do properly.