Why would this happen?
When you've been taught what you're supposed to believe as a Christian, but you haven't personally learned it.
For many, the depth of their faith is as shallow as knowing only what they have been told, what they've heard from others, or from a little reading (very little). Not many actually do the work of learning for themselves what the Bible teaches, and what comprises genuine Christian faith.
When you only know what you've been told, you've accumulated some knowledge, but not to the level of personal understanding. That takes doing some study yourself.
The Bereans were interested in having a correct understanding of the truth by studying the truth themselves. These early Christians had some of the greatest Gospel teachers ever, but that wasn't good enough. They would take what they were taught, and compare that with what they learned through their own study ...
"10 That very night the believers sent Paul and Silas to Berea. When they arrived there, they went to the Jewish synagogue. 11 And the people of Berea were more open-minded than those in Thessalonica, and they listened eagerly to Paul’s message. They searched the Scriptures day after day to see if Paul and Silas were teaching the truth. 12 As a result, many Jews believed, as did many of the prominent Greek women and men," Acts 17:10-12.
It's easier to understand why so many Christians are ill-equipped to explain their faith when studies show us only one in four Christians today open their Bibles outside a church service on Sunday. Many Christians today are too willing to be told what to believe, rather than to learn what God really has said.
In crunch time, only those who have gained an understanding by learning will have a faith that will sustain them.