A few years ago I attended a trading conference where one of the guests speakers was Jack Schwager, author of the must-read Market Wizards series of books. One statement that stuck in my mind from that conference is that a typical learning cycle for a trader is about 7-8 years, which tends to coincide with the length of an economic cycle from boom to bust.

The reason it takes that long is because the market behaves differently at points within the cycle. During certain months or years, high growth stocks may be breaking out to new highs. Other times such as during a topping phase, incremental good news has little upside effect on those same high growth stocks and breakouts may widely fail. A trader needs to learn how to adapt to such different market environments. Over time, the various market regimes become more familiar and after seeing it all, you know how to successfully trade the next cycle.

Here are a few other reasons that I've realized from my own experience:

In order to evaluate the value of your own decisions, you have to wait for the market to tell you the outcome. 

Every trade you place may take days, weeks, months or sometimes even years. Even if you are a day trader and you can get quick feedback on individual trades, it will still take time to evaluate your overall record. 

What's your win/loss ratio? How are your day trades doing in different market environments? Is your holding time frame just right? Day trading opportunities are there for certain moments each day. If you miss them, you have to wait for the next day. Days add up to weeks and months. Trying to nail longer time frames requires even more time.

Studying historical charts and market events is different from experiencing the real thing. 

You can backtest a trading strategy all you want, but it's different from living out the decisions in real time. There's news flow, red and green candles are changing before your eyes, your decisions aren't tainted by your general knowledge of what the overall market will eventually do. In any historical review, you already know the big picture context.

Looking at a historical chart also gives you a distorted sense of time. It's easy to look at the historical prices of a breakout and mentally observe months of data in a single glance. Things look so obvious in hindsight. In real time, you will be offered many decision points regarding your trade as you wait for your idea to play out. Little bumps on the chart suddenly become meaningful, making you question whether you should cut losses, take profits, or add more.

You need a lot of capital to weather the learning experience.

"Market tuition" is what traders often call this. You need money to start off your trading career, and the profits you make at first in your small account may not be enough to cover your living expenses. Trying to find your lifestyle through trading a small account will also affect your trading psychology, making you a more emotional (i.e. bad) trader. 

Mistakes will be made, you will encounter losses as you learn the skill of trading. Sometimes you literally can't afford to withstand such losses. You may have to stop trading for a while to make ends meet. Building up that capital that you need for your own personal security takes takes.

Sometimes people have the privilege of being able to learn trading because they have someone else covering their personal bills. It might be parent, or a spouse, or maybe just a really good living arrangement that pays the rent. If you don't have such an advantage, or worse you are burdened with personal financial obligations, then the road to becoming a professional trader will be a lot longer.

The market changes over time.

Markets go through phases. Sometimes the action is fast. Sometimes it's as exciting as watching paint dry. Those changes in pace of the market can be very confusing. A strategy that worked very well in a bull market may lead to steep losses in a different kind of market. The only way to get used to the changes in market regimes is to trade them and gain experience, and to do that you have to wait for the cycle to run its course.