Why people ask it
Because they are in it right now, and because most environments make it costly to say out loud.
Short answer
Doubt is not the opposite of faith — indifference is. Questions asked honestly tend to produce sturdier belief than questions suppressed, whichever way they resolve.
Deeper answer
The first thing worth saying is that this is common and it is not disqualifying. A great many people who now hold deep conviction went through a period exactly like this. Being told to stop asking is what turns a question into an exit.
It helps to identify what kind of doubt it is. Intellectual doubt is about evidence and arguments, and it responds to study. Emotional doubt usually traces to pain, disappointment or an unanswered prayer. Relational doubt is often about people in a church rather than about God. They are different problems and they need different responses.
Practically: find someone who will not panic, take one question at a time rather than the whole pile, and give yourself permission for the process to be slow. Anyone who cannot tolerate your questions is not equipped to help with them.
Common mistake
Responding with alarm. Panic tells the person their questions are dangerous, which is exactly the message that pushes people out.
Conversation-ready response
""I am glad you told me. You are not in trouble and I am not going to panic. What is the question that is bothering you most right now? Let us just take that one.""
Source notes coming soon.