All taxes are a compromise between what the state wants to collect and what's practical to collect. Property taxes are more practical than most, because the land sits right where it is, it's easy to keep track of who owns it and check that they're paying their taxes. Estate taxes ("death taxes" as they say) are also highly practical, because when someone dies is often the only time we bother to accurately figure out the total value of the estate. We might prefer to tax people in proportion to their total wealth at all times, but it turns out that's very difficult.