-cut-
1. Transaction costs. Every time you make a trade you pay a fee, which works against your profitability. There are brokers that claim to charge no or very small fees, but then they tend to have a wider spread.
2. The psychological aspect, which is more difficult to master the higher the frequency of trades.
3. Time and Experience. Although advertisements and trading houses sell you the idea that with a little training you will be able to start earning money comfortably from home, the truth is that being a successful trader requires a lot of time, study and practice.
4. Mathematical impossibility if you start with a small capital. The idea that you are going to start with $100 and be able to reinvest it until you make it a large capital or that you are going to be able to get a monthly income from it clashes with the mathematics. The smaller the capital and the bigger your aspirations, the more likely you are to lose it all and the faster. Just ask someone who knows real finance, not someone trying to sell you a course or shitposting on bitcointalk.
5. There is no guarantee of success, even with training and practice time. Those who sell you trading courses earn more by selling courses than by trading themselves.
6. Survival bias distorts perception. Success stories are often seen because those who fail simply disappear and do not tell about their experience.
In another thread I commented that Warren Buffet, who is known as the world's greatest living trader, has earned an average annual return of just over 20%, over a sample of decades. He says that if he worked with small capital he could make a return of 50% per year. That's something like 0.136% daily return, but also ask yourself: why is no one as famous as Buffett a day trader? The answer is very simple.