Why one must follow one of the four schools of fiqh

• To be a true follower of the salaf-generation you need to attend one of their schools. If and when you have gone through the curriculum and think you can be an evaluator of the schools' opinions or more, go for it! However, fiqh is like any other field of knowledge: you need to start with some basic facts and move from there. It takes years of study to tell whether a hadith, as an example, really proves an opinion or has been taken out of context. Therefore, as a beginner you are dependent on your teacher's opinions and biases, whether you like it or not, so choose with care.
• The answers of these schools have been evaluated and tested for more than 1100 years since their establishment. This means that the remaining differences of opinion between these schools exist for very good reasons; the right answer cannot be known with certainty. It also means that following the mainstream opinion in any of these schools is much safer than relying on one's own opinion or a person that claims to be independent.
• Relying on one's own opinion is not allowed unless you are a scholar capable of extracting answers directly from their 4 basic sources (Quran, Hadith, Consensus, Analogy). This is very unlikely, because even among the Prophet's companions there were only a handful of them, and they didn't have to evaluate chains of relators with 50 people. A person who draws his own conclusions based on Quran and Hadith is exposing himself to damnation, unless he is truly qualified. The Prophet (may Allah raise his rank) cursed the companions who told a person to take ghusl despite a severe headwound. Surely they made this ruling based on what they knew.
• Even having an opinion of what one thinks is the best answer among the 4 schools requires years of studying. One needs to study Hadith, the science of Hadith, the Quran, quranic interpretation, grammar, liguistics, rhetoric, fiqh, the methodology of fiqh, the general rules of fiqh, etc. Most people who think they can evaluate scholarly opinions have, at best, only a rudimentary knowledge of any of these fields. Indeed, it is rare to find an individual today that truly masters a single one of them.