Why "he seen" doesn't see the light of proper grammar

What kind of error is it?

It is a grammatical error involving the misuse of verb tense.

Why do people make this mistake?

People often make this error due to the influence of dialects or colloquial speech patterns where such constructions are more common, or due to confusion between the simple past tense (saw) and the past participle (seen) that should follow a helping verb.

What is correct?

"Seen" is the past participle form of the verb and should be used with a helping verb like "has," "have," or "had." The correct simple past tense of "see" is "saw."