For so many reasons:
(1) Being a parentified oldest child.Many oldest children are forced to parent/raise younger siblings by parents who are overwhelmed by the number of children they have.Since they cannot effectively raise them, they forcibly enlist the oldest/older daughters to raise them thus forfeiting the childhoods & adolescence of oldest/older children so when they get a chance of freedom,they leave home when the opportunity is available.Usually it isn't so good.They grab the first signs of freedom there is but usually end up MUCH WORSE because they are insufficiently prepared socioeconomically, educationally, emotionally, & psychologically.
(2) Being a throwaway child.There are parents who don't want their children so they will create an atmosphere in which there is LITTLE or NO CHOICE but for the children to leave home.The parents created an unwelcoming home environment in order for the children to leave home as soon as possible.The parents feel that by doing this, they have more space, even freedom.Again there are parents who can't support the number of children they have so they create a toxic atmosphere in which SOME children will leave home, giving them more socioeconomic & psychological parental leverage.
(3) There are children who want to be independent early.They don't want to follow their parents' rules & dictates because they feel that they are more intelligent than their parents.They may even feel that their parents are suitable parents in every sense of the word.hey believe that they can do a much better job by themselves.
(4) They have very strict, authoritarian parents who don't give them any freedom nor independence to think & do things for themselves.These parents are of the school-THEIR way or the HIGHWAY.So children who are more independent & non-conforming are left w/little or no choice but to leave home as soon as they are able.
(5) They have overprotective parents who constantly infantilize them. Such parents believe in protecting & shielding their children from the difficulties & negativities of society.While some overprotective children become excessively mollycoddled & sheltered, others rebel big time, showing their parents that they AREN'T infants anymore & seek independence to the EXTREME.
(6) They are forced to be adults at early ages.There are children who raise & support themselves.Their parents are either little or not involved in their formative years so they feel that they are BETTER OFF w/o the former.