Because religion is laughable

Trinity Explained

Funny Trinity Explained Picture
Funny Trinity Explained Picture - Jesus Christ holding baby Jesus

According to that highly esteemed source of misinformation, Wikipedia, the "Christian doctrine of the Trinity (from Latin trinitas 'triad', from trinus 'threefold') defines God as three consubstantial persons, expressions, or hypostases: the Father, the Son (Jesus Christ), and the Holy Spirit; 'one God in three persons'. The three persons are distinct, yet are one 'substance, essence or nature'. In this context, a 'nature is what one is, while a 'person' is who one is."

If that is as clear as mud, that is because the mysteries of the Divine are beyond your understanding, and you should try a picture book.

Trinity - pigeon picture
 Was that you? It was the pigeon.
Three face Jesus picture
 Three faced Jesus
Are you there dad?  It's me, you trinity picture
 Are you there dad? It's me, you.
Mary trinity explanation picture
The Trinity - I was impregnated by my own child