Insurance isnt worth it when you can afford to replace the item yourself. So if loosing the machine & having to replace it wouldn't bankrupt you or severely impact your life, its not the economical solution in the long run.
Insurance companies are gambling that bad things won't happen to you at rates that pay out more than you pay in on average. They also need to make profit on the transaction & cover their overhead. So if you self insure, in the long run, you pay less overall.
Making payments on a new tractor or buying a new one outright might suck, but if you can reasonably afford to do that if required the odds are you will come out ahead. No guarantees on those odds, but it's better than Vegas.
I have KTAC on mine, but will likely drop it when it's paid off. I never get insurance on any thing under 5 figures... computers, appliances, tools, etc. I can afford to replace them if need be & in the long run I will statistically (not guaranteed) come out ahead. Plus dealing with insurance companies sucks (KTAC MUCH less than the rest though).
Liability is different. Not only are you legally required to have it in many cases (automobile). But most of us can't afford to pay out $100,000 to a million or more. So an umbrella policy for the house & general life stuff or a commercial liability policy makes sense.