Ghost shrimp do not require brackish to grow or breed. Many people get that confused with Amano shrimp, who do need brackish for the babies.
Ghost shrimp are hard to breed due to their larval state they are born in, basically just bobbing around in the water. They easily get sucked in filters, eaten by prey, or by their parents, haha. And after all that, they need microscopic food like infusoria or green water free floating algae. Since we keep our tanks pretty clean, there is very little of that food for them.