It is not a requirement because it is not necessary.
A pass in Air Law can be acheived with absolutely no knowledge of local airspace, airfield layout, circuit procedures and R/T procedures.
Before being sent solo it is a requirement that the student must not be a danger to themselves or otehr airspace users. That requires tyhe instructor to ensure that they have an appropriate level of knowledge.
I can guarantee that if a student has a problem on a first solo and the instructor trys "They have passed air law" as an ass covering exercise, they will soon find that it is meaningless unless the instructor has ensured that the appropriate knowledge regarding the aircraft, the local procedures etc etc has been demonstrated prior to first solo.
Many JAA countries have centralised exams and students sit all the exams in one sitting at a central exam centre. No learning a confuser full of copied authority questions there!. Those countries do not have any problem with students going solo before passing the official exam. However, the instructor must in some cases sign a certificate in the student's logbook stating that they have suficient knowledge to fly solo. They also have to hold an RT licence.
One common reason why instructors push students into doing the exam prior to going solo is to make them do some study as they progress and not end up with all the flying completed but no exams passed.
Next we will hear that som instructor requires the met exam to be passed before solo so that if the sutdent makes a bad decision regarding the weather they can simply shrug their shoulders and say "ah but they had passed the met exam so they shoudl know better".
Regards,
DFC