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.