Mainly because the system is not set up for it, so extending the school day means extra workload and more unpaid hours worked by teachers.
Everything from marking,to staffing ,to resources etc takes a lot of time,money and work. Then you have meetings,assessment,interventions and all the other hats teachers are supposed to wear.
I'm educated in another country and by the end of secondary we had 2 8 hours days and the other 3 were 7 hours . But I have never seen a teacher mark books in their breaks or lunch for example. Many came in just for the hours they were actually teaching and then left. Starting in primary we all had text books for every single subject (so not much planning or resources needed) and two books to write in ,one for classwork ,one for homework. We'd get grades on classwork(homework could be included) and tests ,which did take a lot of time to get through but most teachers did it at home and they weren't that often.
Teachers were just there to teach, whether you think that's a good thing or not that's up to you.
In comparison, my teacher comes in at 7 , gets most things ready for the day, then she runs a breakfast club with my help from 8, 9 teaching starts, kids leave at 3:15. Some days she does an after school club as well. Staff meetings , pupil progress meetings, meetings with this or that parent, finish planning because during her planning session this and that happened, 45 min phone call for that kid, and it's 6 o'clock and she hasn't even looked at the next day and there's still assessments to be done, several books to mark,that to check and email just came in that has a long list of things to do or change.
I'm just a TA and I already find it stressful and busy enough, just the admin ,interventions and actual work bit, without adding all the issues that come with the kids.