Because adolescence, as they are defining it, is not the minimum marriageable age, it is the age at which one can consent to marriage.
Quoting the source I referenced before:
If the marriage is arranged by the father or the grandfather of the girl, then the husband may have sex with her at age 9, according to the Islamic schools that define female adolescence as age 9. The obvious rational for age 9 being Muhammad's example, as recorded in the Hadith.Notwithstanding the precondition of mutual consent, all schools recognise the power of a guardian to marry off his ward before she reaches puberty, and it is not uncommon for this in fact to occur shortly after the female child is born. 32 In this case, the consummation of the marriage must be postponed until puberty.33 The so-called ‘option of puberty’ offers the minor the possibility of objecting to the marriage upon attaining puberty, for as long as the marriage has not yet been consummated.34 However, classical opinions agree that this right of objection can be excercised only through a court and that it is applicable only to marriages concluded by a person other than the father or grandfather of the minor.35