It's not over-insuring. Most people can either add on all their shooting/hunting items onto their standard homeowners insurance. For example, my wife lost 2 iPhones in quick succession and the homeowners policy didn't question the facts, they just paid up.
When it comes to guns, optics and related items, they can often be relatively high value items and if it comes to making a claim (if you blow a gun up in the field due to a blocked barrel, will your insurer pay for a new gun?), you may want to know that the insurer has experience in covering these types of items. Hence looking for insurers/brokers with specialist knowledge of country sports and equipment.
We get liability insurances from the organisations that offer them as part of the membership package. Again, you don't have to join any of them. But if you have an accident and get sued, hope you have deep pockets.