Pure liberalism is no good as a flawless ideal, but faces theoretical as well as practical difficulties. First, the assumption that each person is rational and will make decisions that are in their own best interest is often not accurate. In reality, people are often irrational and make decisions based on emotions, social pressure, or other factors. This can lead to outcomes that are not in line with what a purely liberal system would predict.
Second, even if everyone was rational, the world is not always a fair place. There are often obstacles in the way of people achieving their goals, and not everyone has the same opportunities. This can lead to a situation where the most successful people are those who were born into privilege or who have other advantages, rather than those who are the most deserving.
Third, pure liberalism relies on a strong government to enforce the rules and to protect individual rights. In reality, governments are often corrupt or incompetent, and this can lead to a loss of liberty rather than an increase in it.