Started reloadling with my Dad in about 1962-63. Been reloading ever since. I've asked myself your question many times. I would guess slightly less than 10% reload. Say 7 or 8%. As far as reasons why? I DO know that many are intimidated. Some are eager to to spend $1500-$2000 for a firearm but, think $150-$200 to get started in reloading is TOO MUCH. Some are not detail oriented and find the process of reloading too tedious or time consuming. And, finally, some shouldn't reload and are at least smart enough to know it.
My next door neighbor loves to shoot and has a good income. If he wanted to go shoot, he would go buy a case and go. Several times over the years I asked him what he was going to do if he couldn't buy ammo. He firmly believed that would never happen. He now knows differently. He wants me to load for him. I told him if he could find powder, bullets and primers I would gladly teach him. Of course he can't find that either. He knows better than to ask me for my small stash of components. He's SOL and knows it!