I can see why it is hard to comprehend, because it is a bit like Alice in Wonderland. Much of Educationland makes absolutely no sense. It ought to be possible to complete 25 hours of teaching, with 40 hours of work (although most high performing countries have a ratio closer to 1 hour teaching to 2 hours prep/mark/adjust prep), but actually simply preparing lessons could easily eat up a big chunk of that 15 hours. Making slides and resources, thinking about how to group the students, working out how to make the learning accessible to all students, including those with eg reading difficulties and then recording all this on whatever planning format your school uses, is quite time consuming. In secondary, when there aren't huge curriculum changes going on, you can re-use materials, so you would save time there, but if you are moved to work with a higher or lower set or year group, or you have a particularly strong or weak cohort, or you get a new head who imposes a different planning format, then you are closer to starting from scratch.
Add in team meeting, staff meeting, answering queries from parents and students, dealing with disciplinary and pastoral issues, keeping up with developments in your subject and your 40 hours start to be stretched a bit thin. And that's before we have even thought about assessment. Secondary marking is less relentless than primary, but the peaks are huge. And marking is only part of assessment - teachers are expected to record data on each student, analyse and explain it. This involves looking at lots of different demographic splits and looking at attainment (results) and progress (how far the students have moved) and comparing against targets which will have been based on the child's performance in very flawed tests in year 6 (tests which are undergoing yet more change this year and which are so high stakes for primary heads that in some schools, children are significantly 'helped').
The answer could well be hoping that lots of people like you come into teaching and simply refuse to jump through the hoops. There does need to be a critical mass though, because for now, school cultures and leadership styles make life so unpleasant for anyone who doesn't toe the line, that people roll over or get out.