There is a teaching that is very common in Baptist Churches that the Baptist Church has its origins in the New Testament Church, long before the Reformation. A fair representation of the teaching is found at Providence Baptist Ministries.
A summary of the teaching consists of the following points:
- There have always, since the time of the New Testament Church, been believers that believed in the Baptist/Fundamentalist distinctives including, but not limited to:
- Local congregational authority
- the belief that baptism is not meant for children. It was always a sign of obedience to God after salvation.
- The formal organization of Churches such as the Orthodox and Catholic Church is not found in Scripture. Scripture only teaches about two positions: presbyters (pastors) and deacons
- Throughout history, there have always been believers that held to these beliefs, but they have gone under different names
- Post-reformation, this group, which has existed since the beginning of Christianity, was given a name - Anabaptists, because they re-baptized believers in adulthood because of the rejection of infant baptism. (This is where the Wikipedia version, (and admittedly, commonly accepted version) of history meets the Baptist version of history)
- Later, as is commonly agreed, anabaptists gave rise to the modern "Baptist" denomination and the myriad variants that exist within the "Baptist" moniker.
So @dongle26 isn't simply making things up. This is a very common Baptist teaching. As to the accuracy and veracity, it's debated. While I tend to believe that the individual points are probably accurate, I think most people, when presented with this view, would balk.
It reminds me of what a wise Jedi once said to a young one who had just discovered that some of the things he thought were true weren't necessarily as he thought "You will find that a lot of the truths we cling to depend greatly on your own point of view." Yes, I know Star Wars isn't Christianity, but the saying is applicable regardless.
These points, if true, clearly say that the Baptist distinctive beliefs have existed since long before the Reformation, and that the Baptist name simply was applied to the existing believers later. This could be extended to say that, since the Baptist Church isn't an organized Church with a central authority, in the sense that some other denominations are, the Baptist Church is identified by adherence to certain distinctives. Since these distinctives pre-date the Reformation and stretch back to the original Apostolistic New Testament Church, in a sense, the Baptist Church extends back to it as well.
Of course, as usual, not all Baptists agree on interpreting history this way. The Baptist History and Heritage Society, for example, flat out refutes the teaching. Not all people look at it with the same point of view. Some would call it revisionist history, and some would call it a deliberate misinterpretation of history, or a flawed interpretation of the historical facts. (Which is ironic, considering how strongly the concept of "correct interpretation" is to those of us who self-identify as Baptists and Fundamentalists.)
In general, the "rest of Christianity and the rest of the world" may agree with all of the points, but still say that the "Baptist Church" "originated" at the time the name was applied. Which, also, is true, from a certain point of view. (And in my head, all I can hear is Luke asking incredulously, "a certain point of view?")
So, in summary, this isn't something that is going to be universally agreed on. The best you can hope for is that people say "Yeah, I can see how you came up with that, but..." regardless of which side of this issue you stand on.
Considering that the vast majority of those who call themselves "Christians" is not Baptist, and not even all Baptists agree with either version of history, the majority of "Christians" agree with the "Baptists are Protestants" or "Baptists are products of the Reformation" view. And even though being in the majority doesn't make you right (or wrong), it does affect how the issue is perceived.
As such, the only objective answer to the question of why Baptists are called Protestants is that most of the world sees it that way, right or wrong.