Here's why:

* Metal oxides are generally ionic compounds.

* When dissolved in water, basic oxides react with water to form metal hydroxides, which are bases.

* For example, sodium oxide (Na₂O) reacts with water to form sodium hydroxide (NaOH):

```

Na₂O + H₂O → 2NaOH

```

Note: There are some exceptions to this rule. For instance, some transition metals can form amphoteric oxides, which can behave as both acids and bases.