Why static final int is used in android standard classes avoiding enum?

In standart android classes static final int vars is used to represent different flags. For example, View visability flags:

/** @hide */
@IntDef({VISIBLE, INVISIBLE, GONE})
@Retention(RetentionPolicy.SOURCE)
public @interface Visibility {}

/**
 * This view is visible.
 * Use with {@link #setVisibility} and <a href="#attr_android:visibility">{@code
 * android:visibility}.
 */
public static final int VISIBLE = 0x00000000;

/**
 * This view is invisible, but it still takes up space for layout purposes.
 * Use with {@link #setVisibility} and <a href="#attr_android:visibility">{@code
 * android:visibility}.
 */
public static final int INVISIBLE = 0x00000004;

/**
 * This view is invisible, and it doesn't take any space for layout
 * purposes. Use with {@link #setVisibility} and <a href="#attr_android:visibility">{@code
 * android:visibility}.
 */
public static final int GONE = 0x00000008;

/**
 * Mask for use with setFlags indicating bits used for visibility.
 * {@hide}
 */
static final int VISIBILITY_MASK = 0x0000000C;

private static final int[] VISIBILITY_FLAGS = {VISIBLE, INVISIBLE, GONE};

Why developers avoid using enum in this case?

They do it mainly for performance, as others have said. The main performance impact is the amount of memory they use.

I feel this is the best resource on the matter (from the horses mouth) https://youtu.be/Hzs6OBcvNQE

I know a video is unusual here on SO but it is the most to the point resource on the matter.

A side point: Though Enums are not performance optimised, most apps done in Android Studio should be fine if they use Progaurd to convert all enums at compile time to static final ints. I do this as I find Enums a little more expressive.