Many reasons, most of them the wrong reasons.
However, one reason that I think is justifiable is how Mormons PRETEND to have exactly the same beliefs as ECs, until you ask the right questions... And they don't pretend on purpose, they are taught to do so in seminary school.
You know none of what you've said in this post makes sense. First off, if we were "pretending," it would
have to be on purpose. Pretence implies the desire to deceive. It has never been our desire to deceive anyone. I suppose you really do have to ask the right questions, though, as we are not mind-readers. That doesn't mean we are deliberately trying to skirt the issues or mislead you. It's simply that a lot of Latter-day Saints, particularly those who have always been members of the Church, don't know a lot about Evangelical Christianity and don't know what Evangelical Christians are really asking.
Let me give you an example. If an Evangelical Christian were to ask a Latter-day Saint, "Are you saved?" the Latter-day Saint would likely respond with a confused reply and a blank stare. This isn't because we're trying to play games. It's because, first of all, we almost never speak of "being saved" in the present tense. We speak of "being saved" as something that's going to happen if we "endure to the end." We also use the word "salvation" in different ways, depending upon the context. On one hand, we can speak of "salvation" (meaning "going to Heaven") versus "damnation" ("going to Hell"). That's how I think most Evangelicals use the word. To us, salvation, when used in this way, is almost universal. On the other hand, we speak of "salvation" as a synonyn for "exaltation" which is what we believe Matthew 7:14 is referring to. For us the word has more layers of meaning than it does for you. That's why you have to ask the right questions in order to get the right answers. Some of us Latter-day Saints have learned that, but many haven't.
Your comment about our being taught to pretend we believe the same way Evangelicals do is really unfair. First of all, unless you have attended Seminary, you couldn't possibly know this. More importantly, it's false. I attended four years of Seminary. When we attend Seminary, it's to learn about our religion. It's not to be coached in how to be sneaky in talking to people outside our faith. I have never, ever, ever attended an LDS worship service or class where I have been taught that I should be pretending to believe something I don't really believe, and I believe my experience has been pretty typical. I feel that most of us Latter-day Saints on RF have tried to be completely honest in discussing our beliefs with you. I'm really disappointed to hear you make such an unfounded accusation.