I have known many instructors who have attempted to run ground schools with varied success. Often after lots of effort they run classes and the number of students slowly drops off, or in some cases is never enough students to make it worthwhile. Trying to fit lessons around flying lessons is difficult as the instructor can earn more money flying, and most students have something else they need to do.
Invariably schools don't charge for ground-school, which is why FIs don't get paid for it. Perhaps students need to get used to the idea that a PPL is going to cost an additional £1500 to cover the ground-school, but of course this will significantly reduce the number of PPL candidates which is why schools have not done this in the past.