The thing is, the 11+ over the years has essentially become an 'arms race'.
If nobody did any tutoring, then there would be a level playing field at a certain level, and the absolute results would be lower but the children with the top marks would get in.
On the other hand, in highly competitive areas with few grammar schools, there is now a whole industry of private primary schools that devote time every week to specific teaching towards the 11+ test.
So parents from other schools tutor - themselves or via a paid-for tutor - to try to level the playing field in terms of familiarity, specific skills such as vocabulary, and most importantly speed.
This means that the private school parents are no longer getting the full advantage they feel they are paying for, so they tutor in addition to what the school provides.
Thus the need for tutoring of others increases. Tutors want a year-round income, so they typically recruit new tutees as soon as the previous year group has taken the exam. Many of the best tutors are selective, so there is also a sub-industry of 'pre tutors' who tutor for access to the best tutors....
IME, there is a much higher correlation between 'decent ability, diligent and very highly prepared' and success in highly selective 11+ exams than there is between 'genuinely high displayed ability in school' and such success. However, equally, many children who are tutored still do not get in.