Кастомный инспектор не сохраняет данные

Если хотите изменять приватные поля, тогда сериализуйте их

using UnityEditor;
using UnityEngine;

[CreateAssetMenu(fileName = "ItemData", menuName = "ScriptableObjects/ItemData", order = 2)]
public class ItemData : ScriptableObject
{
    [SerializeField] private Sprite _icon;
    [SerializeField, HideInInspector] private string _name;
    [SerializeField, HideInInspector] private string _description;
    [SerializeField, HideInInspector] private int _maxInStack;
    [SerializeField, HideInInspector] private float _weight;
    [SerializeField, HideInInspector] private bool _isEatable = false;
    [SerializeField, HideInInspector] private int _calories;
    [SerializeField, HideInInspector] private int _water;

    public Sprite Icon => _icon;
    public string Name => _name;
    public string Description => _description;
    public float Weight => _weight;
    public int MaxInStack => _maxInStack;
    public bool IsEatable => _isEatable;
    public int Calories => _calories;
    public int Water => _water;

    [CustomEditor(typeof(ItemData))]
    public class ItemDataEditor : Editor
    {
        public override void OnInspectorGUI()
        {
            DrawPropertiesExcluding(serializedObject, "m_Script");

            EditorGUI.BeginChangeCheck();

            ItemData data = (ItemData)target;

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Name", GUILayout.MaxWidth(80));
            data._name = EditorGUILayout.TextField(data._name, GUILayout.MaxWidth(120));
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Description", GUILayout.MaxWidth(80));
            data._description = EditorGUILayout.TextArea(data._description);
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Weight", GUILayout.MaxWidth(80));
            data._weight = EditorGUILayout.FloatField(data._weight, GUILayout.MaxWidth(40));
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Max In Stack", GUILayout.MaxWidth(80));
            data._maxInStack = EditorGUILayout.IntField(data._maxInStack, GUILayout.MaxWidth(40));
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Is Eatable", GUILayout.MaxWidth(80));
            data._isEatable = EditorGUILayout.Toggle(data._isEatable, GUILayout.MaxWidth(40));
            EditorGUILayout.EndHorizontal();

            EditorGUILayout.Space();
            EditorGUILayout.BeginHorizontal();
            if (data._isEatable)
            {
                EditorGUILayout.LabelField("Calories", GUILayout.MaxWidth(80));
                data._calories = EditorGUILayout.IntField(data._calories, GUILayout.MaxWidth(40));

                EditorGUILayout.LabelField("Water", GUILayout.MaxWidth(80));
                data._water = EditorGUILayout.IntField(data._water, GUILayout.MaxWidth(40));
            }
            EditorGUILayout.EndHorizontal();

            if (EditorGUI.EndChangeCheck())
            {
                EditorUtility.SetDirty(target);
                AssetDatabase.SaveAssetIfDirty(target);

                serializedObject.Update();
                serializedObject.ApplyModifiedProperties();
            }
        }
    }
}