Well, we are cursed in today's litigious age with everyone wanting to sue everyone else for anything they can get. Especially the folks or companies with deep pockets.
So virtually all the flying fields need insurance to protect those who control or own the field from damages that might occur if a accident was to happen.
it only takes one accident to ruin it for everyone at a flying field.
So sorry, AMA provides a service with insurance to help protect those involved. If it wasn't for the insurance we basically would have lost 98% of the flying fields we have now. That would have very likely have been the end of model airplane flying for most purposes here in the USA, probably many other countries with equivalent groups like the AMA too. AMA did get the insurance service and plan many years ago just to prevent the loss of flying fields and actually get us even more places to fly.
For those who refuse to want to take advantage of it for whatever reason, more power to you. You are lucky in having places to fly larger planes than Park Flyers without needing insurance. But if you ever had a accident and wound up being sued for huge sums of money you may come to regret that decision.
Most of us are not that lucky.