Because "to seem" is a stative verb. These verbs are not usually conjugated in the continuous form, since they describe a state rather than an action, and often relate to feelings, thoughts, or in this case, perceptions. Other very common stative verbs are "to like", "to know", that's why it's very rare, if not incorrect, to see sentences such as "I am liking this". I hope it was helpful!

hello can we practice together?