I was an enthusiastically-uber-involved Evangelical Protestant for the first 47 years of my life before converting to Catholicism in 2004.
Many Evangelical Protestants believe that Catholics are not Christians and therefore going to hell. That’s why they try to convert them.
If a Catholic tells them, “Yes, I have accepted Jesus Christ as my personal Lord and Savior,” the Evangelical Protestant will recognize their Christianity.
And it’s true–Catholics HAVE accepted Jesus Christ as Lord and Savior! There is no untruth in this. So I recommend that Catholics use this “salvation language” when confronted by Evangelical Protestants who are trying to convert them. It will help the Evangelical Protestant to understand that you are truly “saved.”
But if the Catholic tells them, "I have been baptized into the Church, and therefore I am a Christian,’ the Evangelical Protestant will say, “No, you aren’t. You have to make a decision to accept Jesus as your personal Lord and Savior.”
I’m simplifying this greatly, and I’m also speaking in generalities. Many Evangelical Protestants are much more knowledgeable and have read enough theology to know that Catholics are truly Christians. In recent years, many Evangelical Protestants have worked side by side with Catholics on various issues like pro-life activities, opposing gay marriage, helping the poor, etc., and they have come to recognize that Catholics are truly Christians.
But there are still plenty of Evangelical Protestants who are separatists and do not recognize Catholics as Christians, and believe that the Catholic Church is a cult. Many of the older Evangelical Protestant apologetic books teach that Catholicism is a cult.
So that’s why the Evangelical Protestants try to convert Catholics to Christianity.
Hope this post helps you to understand.