Because the education system of singapore is an abject failure. Switzerland, a country that the PAP aspires to be like, does not have the number of tuition centres nor force their kids to go thru the tuition hell. The countries that SAF and President scholars like to study in, i.e UK and USA do not have such a tuitoring system, nor subject their kids to hours of homework and tuition after school and on the weekends.
The fact is if the teachers cannot teach all the kids need to know within the school hours, then they have failed as teachers. If the kids have to resort to tuition centres and tutors to understand the school work they were thought in school, its again a failure on the part of the teachers. If students have to do mountains of homework and need the tutors to help them, its just teachings hiding their failings behind all the homework so as to stump the students.