Err, actually, I think you will find most of them say that wicca is an ancestral tradition dating back to Lord knows when, all part of a deception created in the 50s by Gerald Gardner talking a load of crap about what he found in England's New Forest. Believe me, I have talked to enough that hold this view.
The reality is that it cannot be dated to anything earlier than 1902, and in the States only.
It does indeed have historical roots as the orginal idea is an umbrella term of many previous things. IF it was accepted as such there would be no problem but most adherents (and the common idea about it spread by things like Buffy) give it a grand ancestral tradition that is not appropriate.
I am not saying appropriate as pertains to religion. I am saying appropriate as pertains to common sense. Inventing a false history is clearly inappropriate no matter what. I don;t see why the NEED to create such a thing. As you say, all those American Churches are thriving great and they never had to pretend that Europe had been practicing their interpretation of Christianity for thousands of years!
The Wiccans who actually KNOW anything about what they practice acknowledge that it is modern and add that it does not matter. Fair play to them. But too many people hold it in great ignorance.