Because your neighbors have a male. When they go into heat, male dogs will come from all over to try to catch her alone... they will scale fences, dig under.. whatever it takes. Another good reason is to prevent
Mammary cancer, which is very common in female dogs who are not spayed.
Ovarian cancer and cysts also occur in dogs and like in people are very hard to detect because the outward symptoms don’t occur until the growth inside has caused a lot of damage.
Uterine cancer can also occur with the same problem of not being noticed until it is often too late.