Salam alaykum brother and sister
Why do we have to follow the sunnah of the prophet Muhammad and not just follow the quraan ?
Why do we have to follow a specific Mathab?
The sunnah is the example of how to follow the Qur'an.
Madhabs are schools of Islamic law and rulings that were derived from the Qur'an and Sunnah by learned scholars, a madhab is really a set of rulings taken from the Qur'an and Sunnah because we're not able to extrapolate rulings for ourselves due to the fact the majority of us aren't educated in the Islamic sciences.
A madhab is really all the work done for you, someone sat down and examined the Qu'ran and the Sunnah and through their examination they brought forth a ruling for everyday situations that a muslim is commonly faced with, such as how to do wudu correctly, how to pray, how to calculate zakah, what nullifies your wudu, someone examined all the evidence and documented the answer/correct method, so basically all the research has been done for you, whenever a question arises in your head "hmm I wonder what the ruling on such a such thing is" or "I wonder what the method is to do this act of worship", instead of going through all the Qur'an and books of hadith trying to find your answer you can just see the research of a scholar who has all ready done it and presented the evidence from the Qur'an and Sunnah in his madhab.
People follow one because they don't have the means to go through the Qur'an and Sunnah to find the answer to their question and derive rulings, so most people follow a scholar who has done all this, as this is what most people have within their capability to do since they lack an Islamic education.
You don't have to follow a specific one I believe, but you can take the opinions of the scholar of that madhab who has done all the research for you.