I think it's because protestants never had to go through these heresies. They never had to develop their thoughts on it and see all the possible variations of belief about the Incarnation and who Jesus is, and don't realize they cannot all be true and have the same orthodox faith.
It's an argument from ignorance in many cases. IMHO anyway.
only the Catholic Church has been defending the person of Jesus Christ and His Divinity and His humanity for 2,000 years against all types of heretics.
the non-Catholics weren’t there and have no experience in these matters.