Many who have experienced crises of faith in the LDS Church are bewildered at the explanations given by those who have never deeply struggled with doubt in the faith. Often, the explanations given in a typical Sunday School are that individuals were "offended" by a local leader, "wanted to sin" through infidelity to spouse or the LDS Word of Wisdom, or simply became "tired" of having to do all of the things expected of a stalwart Mormon believer.
The confusion arises because often individuals who are struggling with their faith have their own internal experiences and explanations rebuffed by other members who somehow feel as though they know better what the real problem is than the very people they are talking to, or about.
While in the past there have certainly been many "Jack Mormons" who continue to believe in orthodox faith but no longer adhere to orthopraxic practices, in the 21st Century the reasons for leaving are much more complex and nuanced. This has been acknowledged by the highest Church leadership (See Elder Uchtdorf's talk, "Come Join With Us"), but even so the common explanations continue to be shared.
Why is that?
In the end, it comes down to issues of effort and our human response to avoiding difficulties if we can.
We're all humans, and all of us are imperfect. According to Mormon Doctrine, all of us suffer a propensity towards the "natural man" that must be overcome. All of us suffer from causing and taking offense, all of us are guilty at various times of wanting to sin and indulging in it. And all of us have times where we become tired of effort.
The common excuses of why people leave the LDS Church (or at least go inactive) are all basically reformulations of the following: you left because of human failings.
The problem is that because all of us are human and all of us have failings, there is no possible solution to these reasons for why people leave. You can't prevent people from leaving the Church for human reasons without removing the flawed humanity of LDS Church members. It's simply impossible.
What this means, though, is that there is nothing members or leadership of the LDS Church could do to prevent people from leaving. Indeed, they almost have a moral obligation not to take any action, because the issue is our inherent humanity. There is no onus on the membership or leadership to make any changes or to be worried, and the fault for the exit of those who leave lies fully on the doubters themselves.
However, as soon as reasons for leaving are acknowledged that arise from issues that could be resolved to one extent or another, you immediately have shifted some of the responsibility to the membership and even the leadership generally. Any action that they could take to resolve the issues that cause people to leave means they bear some responsibility for those who leave.
If people truly leave for issues such as financial transparency, historical transparency, social justice issues, women's issues, LGBTQ issues, institutional confusion between what is established "doctrine" and what is merely "policy"... those issues have the possibility of at least some resolution and change. Those issues for leaving are not purely the responsibility of those who leave.
That is why the typical answers for why people leave have such sticking power: because they don't represent any responsibility for the believing membership. There is no action to take, no worries to resolve, no lack of knowledge or action to figure out. There is no subconscious fretting about what the inverse of D&C 18:15 might mean: if bringing one soul to the truth brings great joy in the next life, what will be brought to each of us if we play a role, however small, in sending someone away from it?
That is the feeling behind these excuses. That is the reason people can say, "If you don't like the Church you should just leave" without having to consider the theological ramifications of what they've just said to an individual they believe to be a fellow child of God. In the end, believing Mormons can't do anything that would actually cause people to leave, because the only real reasons people leave are all based in actions and situations beyond anyone's real control except for the doubter themselves.
If it's true that people leave for reasons of offense, pride, or unrealistic expectations then it's also true that regular members bear no responsibility for the eternal ramifications of people leaving.
If it's true that people leave for reasons more nuanced, however, that one-sided responsibility no longer exists and members and leaders have to worry about what actions they may need to take, both individually and institutionally, to resolve the problems.
It's just human nature to not want to deal with such difficult issues! ;-)