Why aren't my midwives weighing me at every appointment?

Please ask them. The standard of care is to be weighed at each visit! And hopefully you will be delivering in the hospital, because hopefully not u, but it seems many women from your town want to deliver at home which is not standard of care and a bad idea and I assume your midwife has good md backup.
Do not know. I do not know but it is easy to do and a good way to keep track of inappropriate weight gain in pregnancy.